Should have

Is the version above an alternative to the 3rd conditional below?-- Yes

Also, I read these sentences from a site that said they are perfect infinitives. How are they infinitives?-- They are not; the site was wrong.

English 1b3How are they infinitives?

Bare perfect infinitives, if you insist. They can occur after any modal verb.

should have crashed
would have survived
may have seen
will have done

I think it's just as well to call the whole series of three verbs a verb phrase without messing with the "perfec

Yea, I thought it was to do with the modals creating bare infintives.

So the have crashed is a bare perfect infinitive, but a more accepted label is simply the verb phrase?

Ta

As CJ says, the whole series of three verbs is the verb phrase.

Of those three components, two form the bare perfect infinitive; the modal is the gooseberry.

MrP

Yes, of course-- those little bare rascals are hiding everywhere: Let's go, I can't see, etc.