I know theoretically difference between these two tenses, but it's confusing to me how to know when some action has an effect in the present (for PP)?
I understand all the examples written in grammar reviews, but when I have to use it in my sentence I have no idea what to use.

Today's example: I wrote something in one file, and now I don't know how to say to other person to see it:

"I've left you a file so you can see what I did" - is it correct?
"I tested it and it works for me, so it should work for you either" - is this one correct?

Why does the first sentence go to PP? I don't really know why I('ve) put in PP. Why can be "have done" too, and does it change the meaning of sentece?

I've found an example..

She has broken her arm - the arm is still injured
She broke her arm - the arm is probably OK now

...and I understan it. What's then the meaning of my sentence?

I left you a file - ...?
I have left you a file - ...?

Hi,

Present perfect can be used:

1)    To describe something coming into being or materialized some time in the past and still exists.

I have quit smoking since January 1^st his year. This means I am still refraining from smoking. The effort to quit is still in affect.

But if you say “I quit smoking 5 months ago” in past tense, it carries a connotation that you may have returned to the nasty habit.

To describe an event or action which has been carried out with or without a reference of time.

Have you been to Paris?

Yes I have been to Paris, twice in fact, once on Honeymoon and once on leisure.

Inflation has affected all of us but hit particularly hard on the poor.

Hope this helps, if I didn't confuse you further!

I've done my homework. (use the present perfect tense to show the action has been completed but no time reference is given) 

I've done my homework. (use the present perfect tense to show the action has been recently completed, and the evidence of completing the homework can be seen here and now)

In informal spoken English, people don't always follow these rules.