unless

1.unless=if...not，但语气比if...not重，如unless it is fine tomorrow语气比if it is not fine tomorrow重。

2.以unless引导的从句里的动词用现在时，不用将来时，如unless it is fine tomorrow里用is，不用will be。unless you will see him tomorrow里的will表示意志。

3.下面每句里用虚拟现在式，现在远不及用陈述现在式普通：

• Unless it be (或is) fine tomorrow, we shall not sail.
• Unless he study (或studies) harder, he will lag behind.
• Unless she have (或has) already done her best, she does not deserve the prize.

4.unless=if...not，但下面两句意思不同：

• If he were not my friend, I should not expect his help. （假使他不是我的朋友，我将不期望他的帮助。［他是我的朋友，所以我期望他的帮助。］）
• Unless he were my friend, I should not expect his help. （除非他是我的朋友，我将不期望他的帮助。［他并不是我的朋友，所以我不期望他的帮助。］）

If I were a bird, I could fly和Unless I were a bird, I could not fly都是“我不是鸟，所以我不能飞。”却不可说Unless I were not a bird, I could fly。

但下面每组里的两句意思相同：

• If you had not told me his name, I should not have recognizedhim.(假使你没有把他的名字告诉我,我将不会认识他是谁。)
  Unless you had told me his name, I should not have recognizedhim.
• If he had not arrived at five, he could not have caught the train.(假使他没有在5点钟到达,他不可能已经赶上那班火车。)
  Unless he had arrived at five, he could not have caught the train.

5.可以说You must not use my pen unless writing a letter，但不可说You must not use my pen unless wiping it afterwards。因为第一句里的unless writing a letter等于unless you are writing a letter，第二句里的unless wiping it afterwards等于unless you are wiping it afterwards，讲不通。第二句里的unless该改作without。

6.unless and until作until解。