until ⁶

应当牢记，当until从句与肯定的主句连用时，主句的谓语动词必须是持续动词，而不可能是非持续动词，如上面的see，notice(参见watch/see)。而observe的词义与句意不符。注意下列各句中的主句动词的持续性：

1. Go on until you reach the end. 一直走到尽头。

2. Tigers were hunted and killed until in many areas the entire population had disappeared. 老虎遭到捕杀，以致在许多地区完全绝迹。

3. We must believe that each one of us is able to do something well, and that, when we discover what this something is, we must work hard at it until we succeed. 我们必须相信，我们每一个人都能把某件事情办好；而且，当我们发现这事情是什么的时候，我们就必须努力干下去，直至成功。