Use of Just, Still, Yet and Already:

Just

It is generally used with Present Tense.
It is used to say that something has happened very short time ago.

For examples:-

1. He has just arrived from office.
2. He has just called me.
3. I have just had my lunch.
4. My friends have just gone.
5. He has just told about the meeting.

Already

It is used to refer to something that happened earlier than expected.

For examples:-

Speaker 1: When will the session start?
Speaker 2: It has already started.

Speaker 1: Would you like to watch that movie?
Speaker 2: No,I don’t want to watch it because I have already watched it.

Speaker 1: I will tell you what he has done.
Speaker 2: You don’t need to tell I already know it.

Still

It is used to talk about something which is same as before.

For examples:-

1. It is still raining.
2. He is still angry.
3. She is still worrying about him.
4. She is still waiting for the bus.
5. She is still looking for a good job.

Yet

It is used to talk about something which is expected to happen.
It is generally used in questions and negatives.
It usually comes in the end of the sentence.

For examples:-
1. I haven’t taken my lunch yet.
2. He hasn’t turned up yet.
3. He hasn’t accepted her proposal yet.
4. Have students not completed their homework yet?
5. Haven’t they talked to you yet?