You use yet in negative statements to indicate that something has not happened up to the present time, although it probably will happen. You can also use yet in questions to ask if something has happened up to the present time. In British English the simple past tense is not normally used with this meaning of 'yet'.

You use yet with a negative statement when you are talking about the past, to report something that was not the case then, although it became the case later.

If you say that something should not or cannot be done yet, you mean that it should not or cannot be done now, although it will have to be done at a later time.

You use yet after a superlative to indicate, for example, that something is the worst or the best of its kind up to the present time.

You can use yet to say that there is still a possibility that something will happen.

You can use yet after expressions which refer to a period of time, when you want to say how much longer a situation will continue for.

If you say that you have yet to do something, you mean that you have never done it, especially when this is surprising or bad.

You can use yet to introduce a fact which is rather surprising after the previous fact you have just mentioned.

You can use yet to emphasize a word, especially when you are saying that something is surprising because it is more extreme than previous things of its kind, or a further case of them.

You use as yet with negative statements to describe a situation that has existed up until the present time.