Comparisons: "so [adjective/adverb] as" or "as [adjective/adverb] as"

Question

When learning how to make comparisons, students of English as a foreign language are first told to use the structure "as [adjective/adverb] as". However, at higher levels, they are told that both options ("as/so [adjective/adverb] as") are correct.

I've been wondering if there are any differences between them, but I can't find an answer in my grammar (Cambridge Grammar of English). In a school book, I saw two examples that led me to think that:

1. "as [adverb] as" is possible with both positive and negative sentences
2. "so [adverb] as" is only possible in negative sentences

Is this correct? If so, does it also apply to adjectives?

Examples for adjectives:

• This is as good as that.

• This is so good as that.

• This isn't as good as that.

• This isn't so good as that.

Examples for adverbs:

• Anne plays as well as John.

• Anne plays so well as John. (not correct?)

• Anne doesn't play as well as John.

• Anne doesn't play so well as John.

Answer 1

Yes. So is an alternative to as in this construction in negative contexts, but not otherwise. I don't use it in my idiolect, but many people do.

Answer 2

The rule as I learned it from an anecdote about Walter Lippmann's anger when a writer failed to follow it is this:

Use "as" in positive comparisons, "so" in negative comparisons.

Examples:

• I am as tall as she, but not so tall as he.
• I am hardly so stingy as that, though I am as thrifty as one should be.
• The cars today are not so fast as they once were.