英汉词典

1. （用于否定句中表示某事尚未发生但有可能发生，在疑问句中用来询问某事是否已发生。英国英语中，表示此义时通常不与一般过去时连用）尚（未），还，仍
    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'.

    e.g. They haven't finished yet...
           他们还没有完成。
    e.g. No decision has yet been made...
           尚未作出决定。

2. （与谈论过去的否定句连用，表示当时并非如此）还，尚（未）
    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.

yet的反义词

    e.g. There was so much that Sam didn't know yet...
           还有很多是萨姆当时不知道的。
    e.g. He had asked around and learned that Billy was not yet here.
           他四处询问，得知比利还没到这儿。