GMAT Verbal & Essays

I'm confused of when to use as. I thought as is used for comparing two clauses; however, that's not the case. Sometimes, I see as is used as a modifier. Can someone elaborate a bit?

AS can be used in the following ways

AS I walked, I became more nervous. (=during)
AS I had already paid, I was unconcerned. (=because, since)
AS we did last year, we will exceed our targets this year. (=in the same way)
AS a child, I delivered newspapers. (=in the stage of being)

Just wanted to add a couple more distinctions between "as" and "like," since this comes up relatively often on the GMAT SC.

As can mean "in the capacity of" -- Like cannot.
Correct: Ed spent four years as an accountant.
Incorrect: Ed spent four years like an accountant. (This sentence means
means he was impersonating an accountant, not serving as one.)

Like can create a similarity between two nouns -- As cannot.
Correct: The penguin is like the chicken, in that neither can fly.
Incorrect: The penguin is as the chicken...

And, to add even more, "as" can be used to create a similarity between to nouns DOING two actions, "like" cannot:

He ran (action) as fast as she did.


Does anyone know, by the way, whether in GMAT Grammar the "did" in the sentence above is necessary? The Flashcards from Eric suggest that the "did" can be omitted. Is that true?

According to GMAT and MGMAT Verbal Guide you can drop the "did".