此兩個連詞意義相同。肯定形式表示的意思是"做某事直至某時"，動詞必須是延續性的。否定形式表達的意思是"直至某時才做某事".動詞為延續性或非延續性都可以。正確使用這兩個連詞的關鍵之一就在于判斷句中的動詞該用肯定式還是否定式。肯定句：I slept until midnight. 我一直睡到半夜時醒了。

Wait till I call you. 等著我叫你。(在肯定句中可用before代替。例如 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點才到。

Don’t get off the bus until it has stopped. 公共汽車停穩后再下車。

I didn’t manage to do it until you had explained how. 直到你教我后，我才會做。

區別：1)until可用于句首，而till通常不用于句首。例如：

Until you told me， I had heard nothing of what happened. 直到你告訴我以前，出了什么事我一點也不知道。

2)until when 疑問句中，until要放在句首。例如：

——Until when are you staying?你呆到什么時候?

—— Until next Monday.呆到下周一。

注意：否定句可用另外兩種句式表示。

1)Not until…在句首，主句用倒裝。例如：

Not until the early years of the 19th century did man know what heat is. 直到19 世紀初，人類才知道熱為何物。

Not until I began to work did I realize how much time I had wasted. 直到工作，才認識到我已蹉跎了幾多歲月。

2)It is not until… that… .例如：

It was not until I began to work that I realized how much time I had wasted.