We already mentioned that there is actually no big difference in the spoken language between the imperfect and the perfect. Most likely the present perfect will be used no matter what the regulation would say. Let's make a little test to you associations to see what the difference is.

a) Every day I wrote her a letter.
b) Every day I have written a letter to her.
The first sentence says that at some time in the past and it actually doesn't really matter when it was, I wrote every day a letter.
The second sentence says that my letter writing has some effect on today, very likely I still do the writing and it is something close to me in time, place and/or emotionally.

to Germans these two sentences are just the same, both are correct

a) Jeden Tag schrieb ich ihr einen Brief, aber sie hat nie geantwortet. b) Ich habe ihr jeden Tag einen Brief geschrieben, aber sie hat nie geantwortet.

a) We saw him yesterday.
b) We have seen him yesterday.
In English only the first phrase is correct. The little word yesterday indicates that the present perfect couldn't possibly be correct.

to Germans these two sentences are just the same, both are correct

a) Wir sahen ihn gestern. c) Wir haben ihn gestern gesehen.

We could give you another million of examples the result would be the same. In spoken language there is no difference between imperfect and present perfect. The present perfect is the one used most often in the situation of describing something in the past. It's still to be considered that in written language there are differences to be taken into account.