Sorry, this is going to sound like a lesson, but I am a teacher, and I write a language blog for foreign learners, so that’s how I’m used to doing it. (In fact this has given me an idea for a blog post). This is what I would tell my students:

1. The most common way to talk about result is to use ‘so’. In this meaning (‘as a result’), ’so’ is a conjunction and usually follows a comma:

“He enjoys his job and the salary is good, so he is reluctant to think about moving.”

2a. In more formal language we can use ‘therefore, consequently, thus' and 'hence' with the same meaning as ‘so’. They are adverbs and normally start a new sentence, although 'therefore' and 'consequently' can follow a semicolon and are usually followed by a comma:

“He enjoys his job and the salary is good; therefore / consequently, he is reluctant to think about moving.”

“He enjoys his job and the salary is good. Thus / Hence he is reluctant to think about moving.”

2b. Alternatively, all four can be used after a comma when combined with ‘and’:

“He has both an enjoyable job and a good salary, and therefore / consequently / thus / hence, he is reluctant to think about moving.”

2c. In this sense, despite what lys says, I believe (and dictionaries suggest) all four words to be interchangeable with little or no difference in meaning, the only real difference being in formality. In the dictionary entries below, 'thus', 'hence' and 'consequently' are all marked as synonyms of 'therefore'.

3. ‘Hence’ and ‘thus’ (but not ‘therefore’ or ‘consequently’) can be followed by a noun or noun phrase instead of a clause, in which case they normally follow a comma. In fact 'hence' is usually used like this, but a subject-verb clause is also possible. The meaning is virtually the same ('as a result of this'):

“He enjoys his job and the salary is good, hence / thus his reluctance to think about moving.”.

4a. ‘Thus’ can also be used to mean ‘in this way’, and in this meaning is interchangeable with ’thereby’. They are often followed by an -ing form (present participle):

“He has been given a large salary increase, thus / thereby enabling him to buy a larger house”.

4b. 'Thus' with the meaning of 'in this way' or 'like this' can also be used at the end of a sentence - "You slide the paper in thus", "I spoke to him thus".

5. Hence literally means ‘ from here’, as in “ Get thee hence!,” but like the rest of its family - ‘thence, whence, hither, thither’ and ‘whither’ is not used like this much in modern English. But we do use it to mean "from now " in idiomatic expressions like “Ten years hence”, and the words "henceforth, henceforward".

6. Although those expressions do indeed refer to the future, I can't remember having seen anything suggesting that ‘hence’ in the meaning of 'as a result' only refers to the future or that ‘thus’ only refers to the past. Both these sentence seem fine to me:

“Sales have been good this year and thus we’ll be able to pay out larger bonuses than expected.”
“He spent his formative years in India. Hence his extensive knowledge of Indian culture.”

My problem with lys is that he seems to think that a word like 'hence' or 'thus' can only have one meaning. But dictionaries give these two words at least two meanings each. As far as I'm concerned, in his example sentence - "He was late and therefore missed the bus", any of the other three words (or indeed 'so') could substitute for 'therefore' without any change in meaning (although I'd accept that of the four, 'therefore' is the most natural-sounding).

http://www.merriam-webster.com/dictionary/hence
http://www.merriam-webster.com/dictionary/thus

Well, I'm glad I'm one of the common people and not one of these people who go round telling others what they can and can't do. Even the famous William Safire, who thought this use inelegant, conceded it wasn't incorrect. You'll excuse me if I pay more heed to writers like Jane Austen, Shakespeare, GBS, James Joyce, E.L.Doctorow, Henry James, Kingsley Amis and even Safire on one occasion, all of whom have used the reflexive pronoun without an antecedent. It's people like them who make English a great language, not pedants who shout 'Wrong!' at anyone who says something they don't like.

http://books.google.com/books?id=2yJusP0vrdgC&pg=PA647

@WW - prioritise /proritize is of course a newish verb made from a noun, used predominantly in business, and I think the Z version may be more common here. Other newish businessy verbs made from nouns, like incentivize, also seem to lean towards the Z spelling in BrE.

The changeover seemed to have happened rather earlier than WW2. These graphs are all for British books:

realise / realize - http://books.google.com/ngrams/graph?content=realise%2Crealize&year_start=1800&year_end=2000&corpus=18&smoothing=3

criticise / criticize - http://books.google.com/ngrams/graph?content=criticise%2Ccriticize&year_start=1800&year_end=2000&corpus=18&smoothing=3&share=

Optimise and prioritise are interesting, as they entered British English well after the change, but the results are different: with optimise the 'ise' version was dominant from the start, but with prioritise it's much less clear cut, perhaps because it's so closely connected with business jargon, which has a heavy American influence:

optimise / optimize - http://books.google.com/ngrams/graph?content=optimise%2Coptimize&year_start=1800&year_end=2000&corpus=18&smoothing=3&share=

prioritise / prioritize - http://books.google.com/ngrams/graph?content=prioritise%2Cprioritize&year_start=1800&year_end=2000&corpus=18&smoothing=3&share=

As a serial 'iser', I can't see what laziness has to do with it, I'm merely using the variation which I was brought up with and which is dominant in the quality press and most publishing in the UK.

@Skeeter Lewis - while I agree there is rather a lot of silly anti-Americanism on some of these pages, I don't really see it here. Writing 'honour' rather then 'honor' doesn't strike me as being anti-Americanism unless I start trying to say it's better. The same with 'ise'. And, don't forget, we have a choice; nobody's forcing anyone to use 'ise'.

@Alexander - I more or less agree with your definition of 'thereby', although I'd tend more to 'in this way, in this manner'. But the use in your examples is not how I understand the way we use thereby:

My own version of your example might go something like:

"After weeks of intense debates, the delegates finally came to an accord and signed the Nuclear Disarmament Treaty, thereby establishing a framework for the next round of disarmament."

Here are example sentences from various dictionaries:

"Regular exercise strengthens the heart, thereby reducing the risk of heart attack."

"Diets that are high in saturated fat and cholesterol tend to clog up our arteries, thereby reducing the blood flow to our hearts and brains."

"The aim of the military action was to open the roads to Sarajevo and thereby end the capital's 40-month siege."

"He signed the contract, thereby forfeiting his right to the property."

In each case it means 'in this way'. You could no doubt substitute therefore in all these examples, but I think the meaning would be slightly different - it would simply mean 'consequently'. But interestingly, I think you could replace 'thereby' in all these dictionary examples with 'so', which seems to be able to mean both 'therefore' and 'in this way'.

And I'm afraid I don't think using 'therefore' in your examples make an awful lot of sense:

"After weeks of intense debates, the delegates finally came to an accord and signed the Nuclear Disarmament Treaty, which therefore calls for the immediate and complete abolition of nuclear arms."

Sorry, but I don't really get the logic here. 'The Treaty simply calls for the 'immediate end ...'. It doesn't call for this because the debates came to an end or because the delegates signed the accord. Personally, I wouldn't use either 'thereby' or 'therefore' here.

And I don't think this one works either - "This thing is a balloon, thereby it is made of rubber". My dictionary defines 'thereby' as being - "used to introduce the result of the action or situation mentioned". A balloon is not made if rubber as a result of it's being a balloon, surely? Isn't that putting the cart before the horse?

Sorry to be a bit negative ;), but I think there is a clear difference between 'thereby' and 'therefore'.

I burst out laughing (or perhaps I should say I Iolled) when I read Anonymous's "correction" of Rufus, saying 'gotta' was a contraction of 'got to', not 'have go to', giving the examples:

You've gotta... = you have got to....
You gotta... = you got to...

'You got to ...' is ungrammatical of course, at least in Standard English. And if someone uses 'gotta', are they really going to bother with the 've? - surely it would be something like 'I gotta go', or from my generation - 'I gotta split, man'. (which is perhaps a little ambiguous!)

When I checked a couple of dictionaries, sure enough 'gotta' was defined as "the written form of the word some people use to mean ‘have got to’ or ‘have got a’, which is not considered to be correct". An example of the latter being 'Gotta cigarette? (= Have you got a cigarette)'. Which by Anonymous's reasoning would presumably be (= Got a cigarette?) - which we do say, but is itself an ellipsis of 'Have you got ...'.

And I was thinking of some examples when I realised that there's one case where Anonymous might unknowingly (I suspect) have a point. The well-known expression 'A man's gotta do what a man's gotta do' had just come into my mind, and here indeed we have one construction, third person singular, where 'gotta' does in fact mean 'got to' - 'He's gotta go now'. But one person out of six doesn't really make a rule.

@jayles - OK, we can agree on something, at least. In some contexts, there is very little difference between "have to" and "must", and your example is a good one. But there are some essential grammar points we have to make about when you can and can't use each construction. I teach mainly Upper-intermediate to Proficiency students, and at this level, we really do have to go into some detail. Students want to know.

Interestingly, in Poland, formal English is not the problem, as the use of Polish in business is relatively formal. What's more, words in Polish that are similar to English tend to be from Latin and their equivalents in English are rather formal; it's getting them to be less formal that's the problem. For example, they are much more likely to say "I have observed" than it's more natural equivalent "I've noticed". Informal often sounds more natural and friendly and less stuffy; informal = normal.

I've just noticed (or even observed that) it's -11 C outside!

@HS - do you mean some people pronounce the first syllable as in "break"? I've never heard that.

@HS - Why on earth anyone would want to avoid perfectly good idiomatic English is beyond me, but I suppose it was a joke. Your examples of "must "from South Africa, by the way, are just how "must" is also used in the UK, to show strong or personal advice. But "have got to" and "have to" are more about general obligation, for example to talk about rules and regular obligation.

@jayles - re: emails - most internal emails are written in relatively informal language, so contractions and constructions like "have got (to)" are entirely appropriate.

I presume I'm the one who's "harping on" about the difference between "must" and "have to", so here's one reason why (redundancy be damned): I teach in companies in Poland, and the main Polish verb of obligation is "musieć", which I think you'll agree looks rather like "must", only "nie musieć" doesn't mean "mustn't" but "don't have to". That hardly sounds like a nuance to me. If you think "I must travel to work every day by tram and when I arrive I must sign the attendance register." is natural English, fine, but what about "I don't must wear a tie at work. Yesterday I musted to entertain a new client and tomorrow I'll must go on a business trip"? Nuances? Pah! Most teachers feel a responsibility to their students to teach them English that is both grammatical and natural.

What's more British course books don't "make a huge fuss" about "have got to", they simply let foreign students know that British native speakers will often use this. Many of my students communicate with British colleagues (or Germans who speak English very well), and they have to be aware of these things if they are to understand them. But the students are free to use whichever version they like.

A wide range of vocabulary is great, but not a lot of good if you don't know how to string the words together. Similarly being perfect in grammar is useless without a good vocabulary and a relative fluency in speaking. In fact in TEFL we don't spend huge amounts of time on grammar; it's totally integrated with all the other aspects we need to teach.

@Hairy Scot - Yes, when we want to be more formal or use more elegant language, we use "have", "have to" and standard passive, but in British English, most of us prefer to use good old-fashioned idiomatic "have got" for possession and "have got to" for obligation in normal conversational English. And we can only do so in the present; for everything else we also need to use "have" and "have to".

The same with passive "got": this is an informal construction. But informal is what we use most of the time. As one linguist has put it, "informal is normal". And informal is often also friendlier sounding. I teach students to put in contractions when they are writing informal emails, for example, as uncontracted forms can sound rather stiff. It's a matter of horses for courses.

One or two points about your examples - "have got" is almost always contracted, and "have" is much less so. Which is one of many reasons I don't go for the redundancy argument.

"Must" is not exactly equivalent to "have got to" - it conveys more of a sense of urgency or personal obligation, and the negative "mustn't" is certainly not the same as "haven't got to". That's a really important point when teaching foreigners. But "have to" has exactly the same meaning as "have got to" and their negatives correspond. "Have got to" is simply idiomatic for "have to".

But your last two examples are rather interesting: I think these cases of contracted "have" are perhaps as equally as idiomatic as the "have got" versions, or perhaps even more so. But I think this only happens occasionally.

@Tom - I bet that's not a British course book publisher.