Perfect Infinitives

Perfect infinitives can have the same meaning as perfect tenses or past tenses.

The perfect infinitive is often used to talk about unreal past events.

Perfect infinitives are quite common after modal auxiliary verbs like would, should and could. Note that after a modal auxiliary verb, the perfect infinitive is used without to.

Perfect infinitives in third conditional sentences

Third conditional forms are used to talk about unreal past situations. Here we use a past perfect tense in the if-clause and would + perfect infinitive (have + past participle) in the result clause.

Note that the structure modal verb + perfect infinitive does not always refer to unreal past situations. It can also be used to express our belief that something has happened.