@Skeeter Lewis - My (British) dictionary defines medication as "a drug or another form of medicine that you take to prevent or to treat an illness" - the only difference between them being that medication is uncountable and medicine countable.

The same more or less goes for documentation - "the documents that are required for something, or that give evidence or proof of something". (I don't quite agree with your definition - you still need a verb, such as "provide", so "documentation" must refer to things, not an action, in this meaning). If you're being specific you ask for "a document", but on other occasions "Do you have any documentation?" might be more appropriate. And I would say the same for "leak" and "leakage". If there's one I would say "a leak" but if there's been an unquantifiable amount, I might say "leakage".

Much of it, I think, such as "transport" and "transportation", "medicine" and "medication" is simply down to usage, and especially between BrE and AmE. And I deliberately use the word "usage" instead of "use", because that's the normal word used when discussing language, and for me there is a real difference here. My dictionary again - "the way in which words are used in a language". And some of the main relevant works on the subject: Fowler's "Modern English Usage", "Merriam-Webster Dictionary of English Usage", "Swan's "Practical English Usage", "Garner's Modern American Usage".

All definitions from the Oxford Advanced Learner's Dictionary.

As for "mind" and "mentality" and "reason" and "rationality", surely these are completely different words, and don't get readily confused, apart from in songs perhaps. And if it's Ella Fitzgerald singing, as far as I'm concerned "anything goes", "it's alright with me", because she always sounds "de-lovely".

@Hairy Scot, I'll go along with you about "eventuate" and "apprise", but I think "signage" has a particular meaning for planners and designers which is different from simply saying "signs" - "signs" are specific, "signage" is about signs seen as a group, or related to their design, and could sometimes be a more efficient use of language (notice "use" not "usage" in that context - there is a difference), something I thought you were rather keen on. "We'll have to consider the signage" suggests something more all-embracing for me than "We'll have to consider the signs".

I think your professor has a bit of a problem seeing the wood for the trees. Presumably all he is seeing is "she", and therefore thinks it must be "who".

As to whether to use one or the other, I'm not sure there is any difference, except to say that "what" seems to be more common. I can't see there's a great difference between "losing physical strength" and "losing physical ability", for example, and I would have thought that "what" could have been used equally as well in the two illness examples. In all the examples they really mean "he/she is not the (same) person he/she was".

I tried a comparative search on Ngram, but it's difficult to narrow the search down to exactly your examples. However, a standard Google search brings up some eight million hits for "she's not what she was" and just over two and a half million for "she's not who she was." Unfortunately though, there's a bit of noise, as you get things like "she's not what she was meant to be".

@crumble - I was going to to take you to task and suggest that this wasn't really an example of inconsistency, but on checking words ending in "-ose" you do seem to have a bit of a point - there isn't really a clear pattern, at least not at first glance.

http://www.morewords.com/most-common-ends-with/ose/

But when we dig a bit deeper, there does indeed seem to be a pattern. Apart from choose, all -oose words seem to have a soft "s" - "goose, moose, noose, footloose". I imagine the fact that choose is different might have something to do with the preceding double consonant.

http://www.morewords.com/most-common-ends-with/oose/

But in any case, I would have thought that the difference between "lose" and "loose" was pretty basic, and not exactly an obscure "aspect of the English language".

I also think Sam is right; I don't see many foreign learners making this mistake. And the same goes for "there, their, they're" and "your, you're". But judging by how many times these are dealt with on native-speaker grammar sites, they do indeed seem to pose a problem for some native speakers.

@Skeeter Lewis - interesting point about stress, but I can't really see any difference in stress between "I have to be there at eight" and "I have to go now", where I would suggest that the stress is usually on the main verb in both examples - "be" and "go". Wouldn't we only stress "have" if we were emphasising how important it was?

Yet for me, "I've to be there at eight" works, but "I've to go now" doesn't really.

But in any case, as I said above, I'd use "have got to" in the second anyway, then there's no problem: "I've got to go now".

According to the Oxford Advanced Learner's Dictionary, both are fine, but it suggests "take a decision" is BrE.

http://oald8.oxfordlearnersdictionaries.com/dictionary/decision

Ngram, rather surprisingly I thought (I personally "take a decision"), shows "make" well in the lead in published books in both AmE and BrE, but the difference is considerably larger in AmE.

http://books.google.com/ngrams/graph?content=take+a+decision%3Aeng_us_2012%2Cmake+a+decision%3Aeng_us_2012%2Ctake+a+decision%3Aeng_gb_2012%2Cmake+a+decision%3Aeng_gb_2012&year_start=1900&year_end=2000&corpus=15&smoothing=3&share=

As a linking verb, the verb to be works differently to other verbs. In your first example you have just a simple verb - "rain", and it's that verb you drop in your short answer. But in your second example, as you yourself say, you have a verb and an adjective, and it's the adjective you're dropping. However, this only seems to happen with modals like "would, could, might" etc. In normal tenses, we don't seem to need the "be".

It always rains. I wish it wouldn't. - I wish it wouldn't what? - rain
She's always late. I wish she wasn't. - I wish she wasn't what? - late
but if we said:
She's always late. I wish she wouldn't. - I wish she wouldn't what? - be late
I think that's why we need to add "be" with modals, otherwise it sounds a bit strange: you're not quite sure what the speaker is wishing.

@Hairy Scot - I'm so glad to hear it. Silly me for being a bit ignorant on these matters. And thanks to porshe for your link. It rather reminds me of this "EU directive", which has also done the email rounds:

http://www.musicalenglishlessons.org/jokes/correction/spellingchanges.htm

@Skeeter Lewis - No, sorry but I like my -ise verbs, as evidently do a lot of other Brits. And if anything the trend is going against you. The (London) Times, which use to use -ize, has now followed most other British newspapers in using -ise. Most TEFL books I work with use -ise. "Correct" doesn't only depend on etymology.

And anyway, in BrE you have a choice: if you don't like -ise you can always use -ize, except for the verbs I listed in my previous comment, which are -ise on both sides of the Atlantic.

And Sigurd seems to be correct about -yze/-yse endings, such as analyse (BrE) /analyze (AmE), which according to Oxford Dictionaries are always -yse in BrE (including Oxford English) and -yze in AmE. Thanks Sigurd, I didn't know that.

http://oxforddictionaries.com/words/verbs-ending-in-ize-ise-yze-and-yse-american

@Skeeter Lewis - I'd love to take the glory, but it was the aptly named Percy who got there first. I was just backing him up.

@brandieliz44777 - you're absolutely right, when we are talking about the subject, as in your examples. Strict grammarians would argue that in your examples, only "Mitchell and I" was correct, although many of us would be quite happy to use "Me and Mitchell went to the store" informally. I find it interesting that when we use "I", we usually put the other person first, but when we use "Me" it's usually "Me first".

But in the example given in the question, it's the other way round. "Michelle and I" are not the subject. They are either indirect objects or prepositional objects, depending on your point of view, so those same strict grammarians would insist on "Michelle and me". And many other not so strict grammarians would agree with them. Hence the question, and why the questioner speaks of hypercorrection.

Perhaps a better analogy would be with what some people think to be the classic example of hypercorrection, "between you and I". The argument being that people who say this think it "sounds" more correct, when in fact the technically correct version is "between you and me", as prepositions should be followed by an object form. (Although there may be idiomatic arguments in its favour).

Interestingly RW Burchfield, writing in The New Fowler's, seems to get much more upset about people using the subject form instead of the object form as in "between you and I", than the other way around, as in "Me and Mitchell went to the store".