a) Oh teacher, please, I haven't finished my test yet!
What does it mean?
Which are the elements that you can see in the sentence?
Now, have a look at this one :
b) She has been in Sydney lately.
What does it mean?
If you care to pay attention, you will see that in situation a) you have began your test and it's 10.00 and you haven't finished it yet. Meaning that you began an action in the past that came up to the present moment.
In sentence b) you refer to an action that occured "somewhere" in the past. Meaning that, you give emphasis to What happened rather than to When it happened.
Structure:
have/has + past participle
Time expressions:
already, yet, just, since, for, recently, how long, never, ever...
Just to make sure you have learnt Present Perfect tense:
http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/pperf1.htm
http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/pperf3.htm
http://www.ego4u.com/en/cram-up/grammar/present-perfect-simple/exercises
