Perfect tense

Use of perfect tense. Re-read your grammar books. The perfect is essentially a present tense in English. What follows is necessarily very brief:

If you use any expression that references the past, such as ago, last night, yesterday, on 12 April, in 1987, on Tuesday, last week, a second ago, in times past, once upon a time,  in 2001-05… you may not use the perfect tense (have/has seen), but must use the simple past only (saw). The perfect tense is a special “bridging” tense that pulls past events into the present. The effects of the action must continue to hold in the present – they must still be ongoing. I lived in London for 5 years is completely different from I have lived in London for five years. In the second sentence, you are still living there: the perfect tense essentially describes a present, not a past, reality.