When used with the present perfect.
Still is used to say that something in the present hasn’t finished.
- She still hasn’t arrived.
- I still haven’t bought a ticket.
Already is used to talk about something that happened sooner than expected.
- I have already done my homework.
- It has already been one year since she left me.
Just is used to talk about a short time before.
- I have just done it.
- He has just been told that nothing is ready.
Yet is used to say that something will happen soon. It can only be used in questions and negative sentences. Also, it only talks about the present and cannot be used for the past nor the future.
- I haven’t spoken with him yet.
- Have you visited the Tate Gallery yet?
