Still is used to talk about an action or situation that has not finished.
- She is still working.
- The baby is still asleep.
- We are still waiting for his reply.
- It is still raining.
Yet is used to talk about something which has not happened – it is expected to happen in the future.
- ‘Is Jane here?’ ‘Not yet’.
- He hasn’t come yet.
In questions yet can be used to ask whether something expected has happened.
- Have they arrived yet?
- Is supper ready yet?
Yet is occasionally used in affirmative sentences. In that case it has a similar meaning to still.
- We have yet to receive that parcel. (= We are still waiting to receive that parcel.)
Already is used to say that something has happened sooner than expected.
- We have already finished.
- They have already arrived.
- ‘When is she going to come?’ ‘She is already here.’
In British English, already and yet are commonly used with perfect tenses. In American English, past tenses are often preferred.
- She has already arrived. (GB)
- She already arrived. (US)