That word processor is Microsoft Word, I bet. I get the same thing: I type in something like "The new strategy he had proposed was key to the success of the project" and Word flags "key" as being wrong. It's a flaw in the word processor, because the usage of "key" is correct here.

Oops... how do you edit a comment? Should be "farther", not "father".

I learned that "father" is used with physical distances, while "further" is used elsewhere:

"He drove even farther into Canada today."
"Further research was necessary."

It's just an expression; nothing to fret about. In fact, it has a bit of a humorous feel because the expression is somewhat absurd under analysis. Like "Same old same old."

In this sense (as a "complementizer"), "that" is optional. Using it can make more complex sentences clearer. It can also help make a sentence more formal.

The phrase was certainly given impetus by the old AT&T ad campaign: "Reach out and touch someone."

The logic of "You got another thing coming" is clear in its meaning: something else (unexpected or unwanted) is on its way.

The phrase "If that’s what you think, you’ve got another think coming" is a play on words that incorporates the older term "You've got another thing coming," changing "thing" to "think" for humorous and meaningful effect.

Yes, it bothers me big time. The logical structure of the sentence has a strong natural break before the conjunction. It's certainly a stronger break than what comes after the conjunction. If a comma is place after a conjunction, ideally there should be a semicolon before it.

My biggest peeve is when a comma is placed after "therefore", but not before.

Example: "I completed the project before the deadline and therefore, I started working on another one." Uggghhh!

The term "the late" meaning "the recently deceased" can't be given the "-est" ending, so "the latest" can't possibly have any connotation of death.