Present Perfect

[watdot]

WRONG TITLE! Sorry. It should be PAST PERFECT: I'm preparing to teach a lesson on Past Perfect and am looking for a clearer way to help the students decide when PP must be used. In conversation, it seems that simple Past is often a suitable substitute, although I have found one sentence where it really wouldn't work, and I can't figure out why: When we got out of the hall, I found that I had left my purse on the seat. Any feedback on this would be very welcome! Thank you.

[Tdol]

After verbs like find out/realise/remember the use of the past and past perfect is different:
I found I didn't like it- not liking came later
I found I had forgotten it- forgetting came earlier.

[Mister Micawber]

.
Generally, past perfect is required only if the chronological order of the two past events is unclear. Otherwise (which is usually, because we use the words 'before' and 'after' so often), two simple past tenses will suffice. In addition, it is really only in the formal written language that the use of present perfect is paid much attention to.

Tdol's case is indeed one of the few where it still makes a significant difference.
.

[rewboss]

When you use the perfect aspect, you're focussing attention on the result of the action, not the action itself. Compare:

"I made a cake, and then we ate it." (A simple retelling of past events)

"I've made a cake -- would you like a piece?" (Here, we are focussing on the fact that there exists a fresh homemade cade)

In the present perfect, the result of the action is still present. In the past perfect, the result of the action is also in the past.

"I finished making the cake, and then Peter arrived."

"I had finished making the cake when Peter arrived."

In the second sentence, we are emphasising the result of making a cake -- either the cake itself, or your exhaustion, whichever is more important for the discussion.

Here's another example:

"She had been to London before, but when she was there last year she didn't recognise the place."

The "result" of the action of going to London before is the expectation that she will recognise the place -- her memories of the London of old. This "result" was significant in the past, that is, last year. If, on that visit, she had asked someone for directions, she might have said, "I have been here before, but..."

Note that American English prefers to avoid the perfect aspect. As always, your mileage may vary.