A practical english grammar - Vyssaja skola 1978
Echo verbs
Sound symbols
The auxiliary verbs be, do, and have, often stand alone and replace a predicate (an ordinary verb or verb phrase, along with all its complements and modifiers) which has already been introduced in a context. Used in this way, they are called echo verbs.
My wife buys her clothes in New York.
My brother’s wife does, too.
The echo verb does in the second sentence replaces the entire predicate of the other sentence, not just the verb buys.
The children haven’t seen the Washington Monument, but I have.
The echo verb have in the second part of the sentence replaces the entire predicate of the first half, except for the negative n’t.
This structure is a subtle and rather difficult point in English verb grammar, and it must be thoroughly mastered. Echo verbs are used as often as possible in English, since it is unreasonable and unnecessary to repeat whole predicates when the meaning is clear without doing, this. (See also Chapter 20.)
They are working. We are, too.
She doesn’t know it. I don’t, either.
I haven’t seen him. Maybe Charles has.
I know it. She does, too.
They knew it. The others didn’t.