What is meant by "present perfect", "past perfect", and "future perfect" verbs? How can I identify them or know which is which?
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Jimi Oke's answer is useful, but incomplete in an important respect.
Perfect tenses indicate completion with continuing relevance (to now for the (present) perfect, but to other reference points for the other tenses).
The simple past indicates that the action is complete and finished with (eg there is no longer any possibility of doing the action).
The (present) perfect indicates that the action is complete, but is somehow still relevant now - in its effects perhaps, or in the possibility of repeating it.
So "I saw him" implies that the possibility of seeing him again has passed - perhaps he has left; whereas "I've seen him" suggests that he is still here.
I believe that this distinction is less clear-cut in US than UK English, though the difference has been decreasing. Thirty years ago it was rare in the UK to use the simple past for very recent events (so "Did you eat it?" would mean this morning, or yesterday, but not just now), but that has been changing.
The perfect tense indicates completion.