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:
- "as [adverb] as" is possible with both positive and negative sentences
- "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.