Present perfect and past simple

Present perfect: have/has + past participle (studied, been, ...)
Past experiences.
I've been to London, but I haven't been to Paris.
She's never met his ex-wife.
Have you ever lost your mobile?
We often use ever and never when we ask or talk about past experiences. They go BEFORE the main verb.
Recent past actions.
He's just arrived at the room.
With yet and already (for emphasis)
I've already done my homework. 
Have you finished yet?
My brother hasn't found a new job yet.
Just and already go BEFORE the main verb in positive sentences, yet goes AT THE END of the phrase in negative sentences and questions.
Unfinished states (non-action verbs) which start in the past and are still true now.
They've known each other for 10 years.
How long have they been married? Since 2003.
Use How long...? + present perfect to ask about an unfinished period of time (from the past until now).
Use for+a period of time, or since with a point of time.

Present perfect or past simple?
Use the present perfect when there is a connection between the past and the present.
Use the past simple to ask or talk about finished actions in the past, when the time is mentioned or understood. We often use a past time expression.
I've been to Barcelona twice.(= in my life up to now) I went there in 1997 and 2009. (= on two specific occasions)
How long have you been married to Jane? (= you are married to Jane now) How long were you married to Jane? (= you are not married to Jane now)
I've bought a new computer. (= I don't say exactly when) I bought it on Friday. (= I say when)