Present Perfect or Simple Past?

[Christie]

Hello people! I dont understand the difference between these two tenses.
When I use Present Perfect and when Simple Past?
Could somebody please answer me that?

If I have action which happened long time ago, but I dont know actually when, do I say- I was in London or I have been in London?

I know that Present Perfect rarely have verbs of frequency, but also it refers on present action. So, in this case, I was in London once and actual time of that event is not well known to other people. So what tense I should use? Or if I use in CV application, one after another sentences-
--1. I worked as a secretary for 12 years, than I work as a nurse for 3 years. Should I write that in Simple Past or Present Perfect?

And one more thing that I dont understand.. A,an, and The articles. I cant in no way understand it. Once you use the, once a or no articles. I have sentence.- I work in the classroom which have a blackboard and the desk coloured in green. So why is it the classroom, a blackboard and than again the desk. What does it mean on clear way? I am really hope that someone will answer me and solve my confusion about this matter.

[Edison_Chen_e_c]

I was in London. (definite time point)
I have been in London. (I have been in London since~) or (indefinite time point)

So in my opinion in your subject sentence you want a indefinite time, which may cause me to use perfective. But you need contect to make it clear or otherwise it's not appropriate to say only " I have been in London."

<cf>:
I just wrote the letter in Eng. (finishing the act at the definite time point)
I have been written the in Eng. (in the indefinite period of time, but past)

I lived in London for ten years. It's fine.