Besides simple infinitives like (to) write, there are also progressive, perfect and passive infinitives.
Form: (to) + be + present participle
Examples are: (to) be writing, (to) be reading, (to) be walking etc.
The progressive infinitive is used to suggest that actions and events are/were/will be continuing around the time we are talking about.
- I happened to be waiting for the bus when the accident happened.
- You must be joking.
Form: (to) have + past participle
Examples are: (to) have received, (to) have broken, (to) have seen etc.
Perfect infinitives can have the same kind of meaning as perfect tenses or past tenses.
- I am happy to have left school. (= I am happy that I have left school.)
- You seem to have annoyed him. (= It seems that you have annoyed him.)
- It is nice to have finished work. (= It is nice that I have finished work.)
We often use perfect infinitives to talk about ‘unreal’ past events.
- If you had run a bit faster, you would have won.
- You should have told me you were coming.
Form: (to) be + past participle
Examples are: (to) be given, (to) be seen, (to) be loved etc.
Passive infinitives have the same kind of meaning as other passive forms.
- Everybody needs to be loved.
- There is a lot of work to be done.
- She ought to be told about it.
Perfect progressive infinitive
Form: (to) have been + present participle
Examples are: (to) have been crying, (to) have been waiting etc.
- We have been waiting for ages.
- How long have you been living here?
Perfect passive infinitive
Form: (to) + have been + past participle
Perfect passive infinitives are also common.
- They were lucky – they could have been killed.