Past modals tell what could have, would have, and should have happened. To form these past modals, use could, would, or should followed by have, followed by a past participle verb. Use have for all pronouns; never use has or had to form a past modal.

What tense is could have?

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.

Would have could have should have idiom?

Used to dismiss one's or someone else's regrets or worries about past actions or the lack thereof.