These words tell you what tense you have to use. For the Present Perfect the following words are used quite often:
JUST
-I have just finished doing exercise. (in positive)
YET
-Have you finished your homework yet? (in question)
-We haven’t finished our work yet. (in negative)
NEVER
-They have never studied English. (in negative)
ALREADY
-She has already had his breakfast. (in positive)
EVER
-Have you ever been to the USA?
SO FAR
-What have you done so far? (in question)
UP TO NOW
-How many letters have written up to now? (in question)
RECENTLY
-What have they done recently? (in question)
SINCE
-I’ve been a teacher since I was 26 / since 2004. (in positive)
FOR
-Mike has been a doctor for twenty years. (in positive)