Still v/s yet v/s already

:

1. Still - used to talk about something that hasn't finished especially when it was expected to finish earlier. 

- used in questions, affirmative and negative sentences

He is still angry at his best friend.

Are you still smoking? I though you had quit.

2. Yet - used to talk about something which is expected to happen

- used in questions and negative sentences.

Have you written the report yet?

She hasn't finished her homework yet.

3. Already - used to talk about something that has happened, often earlier than expected.

- used in affirmative sentences and questions

I already know what I will buy you for your birthday.

Has he already left the party?