PRESENT PERFECT:
We use this tense both for finished and unfinished actions. Unfinished actions are those who start in the past and still continue in the present . We normally use it to say 'how long' an action or state has continued with 'since' and 'for'. Have a look at these examples:
.- I've studied piano for two years (I am still studying piano right now)
.- She has been married since 2006 (She is still married at present)
The difference between since and for is that "Since" is used + a specific time: since March 3rd, since 2006, since twelve o'clock ... and "For" is used with "A period of time": For two years, for six months, for eight hours, for a long time ...
Finished actions include the following uses with the present perfect:
.- Unspecified time: If we do not know when something happened we should use the present perfect:
I've hurt my leg.
.- Unfinished time words (this month, this afternoon, today ...)
I've seen her twice this week.
.- Present result: A finished action which focuses on the result on the present:
Tom has lost his car keys (so, he can't come to school today)
.- Life experience: Usually with ever or never:
.- They have never been to Germany

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