A practical english grammar - Vyssaja skola 1978

Infinitive echo with TO alone
Sound symbols

Just as finite verbs can be echoed in various ways, as we showed in Chapter 6, infinitives can be echoed by omitting the base form of the verb and retaining only the word to. The context tells which verb is understood to follow to. The expressions we are studying in this lesson are used frequently in this way, making for a tremendous variety of possible verb echoes.

Do you work for that company?

No, I don’t, but I used to.

Are you going to study now?

Yes. I have to.

Are you going to Mary’s party?

Well, I’m supposed to, but I’m not certain that I will.

Did your friends arrive yesterday?

No. They were to, but they didn’t get here.