d. after certain adjectives.

It's difficult to explain how to get there. It's possible to walk there.

e. after make and let.

She made me do the exercise again, (active – without ‘to’)

I was made to do the exercise again, (passive – with ‘to’)

He let me borrow the car. (active - without 'to')

I was allowed to borrow the car. (‘Let’, in the sense of ‘allow’, is not possible in the passive.)

f. to express purpose.

I came here to team English.

I need more money to buy the things I want.

g. after certain verbs followed by question words, e.g. what, where, who.

I didn't know what to do.

Can you tell me how to get there?

Show me where to put it.

Do you know where to buy it?

After these verbs and others with similar meanings, it is possible to use how, what, where, when, whether etc.

ask consider explain wonder find out understand

Forms of the infinitive

1 The continuous infinitive

The continuous infinitive is formed with to be + present participle.

It expresses activities in progress.

I'd like to be lying in the sun right now.

He seemed to be having financial difficulties.

2 The perfect infinitive

The perfect infinitive is formed with to have + past participle.

I'd like to have seen his face when you told him.

He seems to have forgotten about the appointment.

3 The passive infinitive

The passive infinitive is formed with to be + past participle.

I'd like to be promoted to sales manager.

I asked to be informed as soon as there was any news.

Notes

The continuous, perfect, and passive infinitives can also be used with modal auxiliary verbs, but with these verbs to is omitted.

You should be working, not watching television.