Could has no tenses, no participles, and no infinitive form. There is no past tense, but could have followed by a past participle is used for referring to something in the past that was not real, or something that may possibly have been real: I could have been killed.

What tense is could have?

These past tense modals are useful for expressing your present feelings about a past decision (or other action). Could have, would have, and should have are sometimes called “modals of lost opportunities.” They work like a grammatical time machine. The simple past just tells what happened.

What is the present tense of should have?

Should' is the past tense of the word 'shall. ' When using the words 'should have' you are talking about something in the past that you 'ought to' or 'might have' done.