A traditional usage rule draws a distinction between comparisons using as … as and comparisons using so … as. The rule states that the so … as construction is required in negative sentences (as in Shakespeare’s “’tis not so deep as a well”), in questions (as in Is it so bad as she says?), and in certain if clauses (as in If it is so bad as you say, you ought to leave). But this so … as construction is becoming increasingly rare in American English, and the use of as … as is now entirely acceptable in all contexts.
as … as and than. When making comparisons involving both as … as and than, remember to keep the second as in formal style. Write He is as smart as, or smarter than, his brother, not He is as smart or smarter than his brother, which is considered unacceptable in formal style.
as instead of that. In many dialects, people use as in place of that in sentences like We are not sure as we want to go or It’s not certain as he left. But this use of as is limited mostly to speaking. You should therefore avoid it in formal writing.
as instead of that or who. Some nonstandard varieties of American English differ from the standard language in the form and usage of relative pronouns. Where Standard English has three relative pronouns—who, which, and that—regional dialects, particularly those of the South and midland, allow as and what as relative pronouns: “They like nothing better than the job of leading off a young feller like you, as ain’t never been away from home” (Stephen Crane). The car what hit him never stopped.
as meaning “because” or “when.” When as expresses a causal relation, it should be preceded by a comma, as in She won’t be coming, as we didn’t invite her. When as expresses a time relation, it is not preceded by a comma: She was finishing the painting as I walked into the room. When you begin a sentence with a clause that starts with as, make sure that it is clear whether as is used to mean “because” or “at the same time that.” The sentence As they were leaving, I walked to the door may mean either “I walked to the door because they were leaving” or “I walked to the door at the same time that they were leaving.”
as used redundantly. As is sometimes used superfluously with verbs like consider and deem. For more on this issue, see redundancy under Style.
as in parallel constructions. Constructions of the as … as … form are sometimes difficult to keep parallel. For an explanation of this problem, see parallelism under Style.