We use Present Perfect
1. to talk about actions that happened in the past and we don't say when these actions happened.
The exact time is not mentioned because it is not important or not known.
They have bought a new car.
(When did they buy their car? It is not important. What is important is the fact that they have got a new car.)
2. to talk about actions that have recently finished and their results are visible in the present.
Harry has broken a cup.
(We can see a broken cup on the floor.)
3. to talk about actions that started in the past and are still continuing in the present. We use "for" and "since" in these sentences.
Ann has been a doctor since 2015.
(Ann started working as a doctor in 2015 and she is still a doctor.)
Ben has been a teacher for ten years.  
(Ann started working as a teacher ten years ago and she is still a teacher.)
4. "have gone to" or "have been to"
They have gone to London.
(This means that they are still in London. They haven't come back yet.)
They have been to London.
(This means that they have visited London. But they are not there now. They have come back.) 
5. with superlative adjectives
That’s the best film I have ever seen.
just They have just seen  him.
already They have already seen him.
ever Have they ever seen him?
never They have never seen him.
always They have always seen him.
They haven’t seen him yet.
Have they seen him yet?
still They still haven’t seen him.
lately/ recently
They haven’t 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.
before They have seen him before.
since They haven’t seen him since June.
for They haven’t seen him for 6 months.
How much (How many) How many times have they seen him?
How long How long have they seen him?
1. We usually use the words just/ already/ ever/ never / always before V3.
I have just spoken to Carl.
They have already 
done this task.
Have they ever been to New York?
She has never seen this film.
I have always worked hard.
2. We use the word yet at the end of the negative or interrogative sentences.
They haven’t finished their project yet.
Have your parents arrived yet?

3. We use the word still before "have/has" in negative sentences.
I still
haven’t done it.
V3 — Verb in the 3rd form (Past Participle). E.g. written, gone, listened, etc.
Revise irregular verbs here and how to add "ed" to regular verbs here.