Here Comes the Tenses! - 6 Present Perfect

• about a period of time which is continuing; for unfinished actions and events; time stated is not finished (with since, for, still, yet)
  I've practised my piano since 4 o'clock. (it's 4:15)
  I've lived there for 4 years. (still living there)
  I still haven't finished my homework.
  Mum hasn't prepared dinner yet.
  The builder have started working on the kitchen this week.
  

• express recent event (often with just); to focus on the results of an activity or event which is complete
  Jack has just been to the bank.
  I've mentioned the car. (= I've just finished so we can go out in it now)
  

• unspecified past even or time is irrelevant; talk about how many times something has happened (with adverbs like already, before, ever and never; after a superlative)
  I've lost my new camera. (it's irrelevant when)
  Pete has been to New York 3 times.
  I've met her before somewhere.
  It's the biggest dog I've seen.