此两个连词意义相同。肯定形式表示的意思是"做某事直至某时"，动词必须是延续性的。否定形式表达的意思是"直至某时才做某事"。动词为延续性或非延续性都可  以。 正确使用这两个连词的关键之一就在于判断句中的动词该用肯定式还是否定式。
    
    肯定句：
    I slept until midnight. 我一直睡到半夜时醒了。
    Wait till I call you.   等着我叫你。
    （在肯定句中可用before代替 Let's get in the wheat before the sun sets.）
    
    否定句：
    She didn't arrive until 6 o'clock.
    她直到6点才到。
    Don't get off the bus until it has stopped.
    公共汽车停稳后再下车。
    I didn't manage to do it until you had explained how.   直到你教我后，我才会做。
    
    1)Until可用于句首，而till通常不用于句首。
    Until you told me, I had heard nothing of what happened.
    直到你告诉我以前，出了什么事我一点也不知道。  
    2)Until when 疑问句中，until要放在句首。
    ---Until when are you staying?  你呆到什么时候？
    --- Until next Monday.  呆到下周一。  
    
    注意：否定句可用另外两种句式表示。
    (1）Not until …在句首，主句用倒装。
    Not until the early years of the 19th century did man know what heat is.
    直到19 世纪初，人类才知道热能是什么。
    Not until I began to work did I realize how much time I had wasted.
    直到我开始工作，我才认识到了我已蹉跎了几多岁月。
    (2)  It is not until… that…