A temporal feature of the verb which denotes the speaker's standpoint of the event described by the verb, as from outside of the event and seeing it as a completed whole.
A grammatical form (often a verb form) that refers to an event, transaction, occurrence, or object that happened (or had happened), or existed, at some time before now (the applicable reference time).
A grammatical form (often a verb form) that is used in a conditional expression to refer to an event, transaction, occurrence, or object that is hypothetical and often counterfactual.