Could someone explain (in simple terms) which of the following are correct, and if they are correct, when to use them? Thanks.
Is this just a memorization thing? Thanks.
Has seen means that the event (seeing) started and ended at least once before now. This statement compares the past to the present. The event happened at Time A, it is now Time B and we are talking about the present (Time B).
Had seen means that the event (seeing) started and ended in the past at least once before a second, more recent, time or event in the past that has now ended. This statement compares the past to the past. The event happened at Time A and ended before Time B. It is now Time C and we are talking about Time B.
Example: Jerry saw the dog once per day on Monday, Tuesday, and Wednesday. On Wednesday, the dog chased him. If it is now Friday, we can say that
Had saw and has saw are both incorrect. This is not just a memorization thing. Whether the event happened in the past relative to another in the past or relative to the present time determines which form to use.