An auxiliary verb (be, have, can, will, would, etc.) can be used instead of a whole verb group or instead of a verb in order to avoid repeating words from a previous clause.
They say they have already chosen a candidate for the job but I don’t think they have.
- Would any of you be happy to live alone? - I would.
If the auxiliary verb in the previous clause or sentence is complex, then we can leave out everything except for the first auxiliary verb:
- They could have been stuck in traffic. - Yes, they could.
If there is no auxiliary verb in the previous clause or sentence, or if the auxiliary is a form of do, we can use a form of do instead of repeating the main verb. We use do when the main verb is used in present simple form and did when it is a past simple form:
Laura goes to the gym twice a week, and I do too.
- I didn’t tell anyone about it! - Relax, no one thinks you did.
If be is the main verb in the previous clause or sentence, we must repeat it:
- Your brothers are so noisy. - Yes, they always are.
If have or have got is the main verb in the previous clause or sentence, we can usually use a form of either do or have:
- Do you think I have any chances to get this job? - I bet you have. (or I bet you do.)
However, if we use have + noun in the previous clause or sentence to talk about actions (have a shower, have a shave, have a good time, etc.) we prefer do:
I was sure I wouldn’t be able to have a good time, but I did.
If we use have as an auxiliary verb, we can often follow it with done instead of repeating the main verb. This happens particularly in spoken English:
- She has never said anything bad about me. - In fact, she has (done) this time.
However, this is usually not possible when the verb being substituted is intransitive:
- Has he already gone? - I think he has. (not has done)
Similarly, after a modal auxiliary verb (can, could, may, might, must, ought to, shall, should, will, would) we can use do, particularly in spoken English:
- Will you be seeing them today? - I might (do).
Sometimes we can use be instead of do with a similar meaning:
- Will you be seeing them today? - I might do / be. ( “do” replaces “see them today”, “be” replaces “be seeing them today”)
If we use be as an auxiliary verb in the previous clause or sentence, we can use be after a modal:
- Is Tom staying the night? - No, I think he won’t (be).
However, if be is used as a main verb in the previous clause or sentence, or as an auxiliary verb within a passive structure, we can usually leave out be after a modal in informal contexts only.
- Jackson is going to be late. - I knew he might (be). (informal)
Leaving out to-infinitives
“To” is sometimes used instead of a clause beginning with a to-infinitive when it is clear from the context what we are talking about:
I wish I could visit you, but I won’t be able to.
It might have been a better idea to give her kids more freedom but she chose not to.
However, when we use the verb be in the previous sentence or clause the to-infinitive form of be is repeated in the next clause or sentence:
Jessica was absolutely furious - I never thought she could be.
After most nouns and adjectives that can be followed by a to-infinitive clause, we can leave out the to-infinitive clause or use to:
plan, chance, idea, opportunity, promise, suggestion; afraid, delighted, determined, willing
I’m not going to move from this apartment. At least, I don't have any plans (to).
- Will you be able to make it up with him? - Well, I’m determined (to).
We can also leave out a to-infinitive or use to with some verbs:
to promise, to forget, to agree, to ask, to begin, to refuse, to start, to try
- Why didn’t Richard visit the theatre with us? - He refused (to).
- Will Lizzy pick me up? - Yes, she promised (to).
We don’t leave out to after verbs which can’t be used without a complement:
to mean, to advise, to afford, to be able, to choose, to deserve, to expect, to fail, to hate, to hope, to intend, to love, to need, to prefer
- Will you go to Malta next year? - I hope to.
I admit that I offended you but I didn’t mean to.
After want and would like in if-clauses and wh-clauses, we can either leave out a to-infinitive or use to:
You can have another piece of cake if you’d like (to).
Take a seat if you want (to).
You can come whenever you want (to).
In other clauses (not if- and wh-clauses), we include to:
I was planning to spend an evening together with her, and I would still like to.
I was there because I wanted to, not because someone forced me to be there.
In if-clauses and wh-clauses, we usually leave out to after like:
- These cupcakes look so delicious! - You can have one if you like.
You can leave whenever you like.
NB! We include to with negative forms of want, would like, and like, including in if-clauses and wh-clauses:
- Shall we go for a walk? - I don’t really want to.
I should have told you before but that would have made you upset so I didn’t want to.
She wouldn’t mind you asking for help. - No no, I wouldn’t like to. I’ll manage.
