Is it seven o’clock already?

(The speaker didn’t expect it to be so late.)

Is it seven o’clock yet?

(The speaker thinks that probably it’s almost seven o’clock.)

We use still not yet or already to refer to the continuation of a situation:

Negatives with yet mean that something has not happened up to now:

Negatives with still suggest that the situation should have changed, but it has not:

Negatives with already are far less common than negatives with yet and still. They usually refer to things which should have happened before they did happen:

now and then

between now and then