Theory:

TIME EXPRESSIONS used with Present Perfect:
 
just
They have just seen him.
already
They have already seen him.
ever
Have they ever seen him?
never
They have never seen him.
yet
They have not seen him yet.
 
Have they seen him yet?
still
They still haven't seen him.
since
They have not seen him since June.
for
They have not seen him for 6 months.
lately / recently
They have not seen him lately / recently.
this morning
(this week, today...)
They have seen him this morning.
once
(twice, three times...)
They have seen him three times .
so far (up to now)
They haven't seen him so far.
How much (How many) 
How many times have they seen him?
How long
How long have they seen him?
before
They have seen him before.
We usually use the words just / already / ever / never before the past participle (3rd form of the verb).
Example:
I have just  spoken to Carl.
They have already  done this task. 
Have they ever been to New York?
She has never  seen this movie.
We use the word yet at the end of the negative or interrogative sentences.
They have not finished their project yet
Have your parents arrived yet ?
We use the word still before "have / has" in negative sentences.
Example:
I still haven't done it.