＝if...not...，意为"除非，如果不"。

I will not go to their party unless I am invited.(＝I will not go to their party if I am not invited.)

假如他们不邀请我，我不会去参加他们的聚会。

二、让步状语从句

让步状语从句表示前后句之间为让步关系。让步状语从句常由以下从属连词引导：though，although，as，even if/even though(即使)，疑问词－ever/no matter＋疑问词(无论......)等。

1.though，although，as，while引导的让步状语从句

这些词引导让步状语从句时，意为"虽然，尽管"，although 较为正式。

Although/Though he failed many times，(still) he kept on trying.

尽管失败了很多次，他还是继续尝试。

While I like the color，I don't like the shape.

尽管我喜欢它的颜色，但不喜欢它的形状。

[温馨提示]

(1)though和although引导的让步状语从句可放在主句之前，也可放在主句之后，不与but连用，但可与表示转折的yet，still连用。

(2)though引导的从句可以倒装，也可以不倒装，但as引导的从句一定要倒装，而although引导的从句不倒装。倒装形式为"形容词/副词/名词(不加冠词)/动词原形＋though/as＋主语＋谓语"，即将形容词、副词、分词、名词、动词原形置于句首。

Tired as/though he was，he continued working in the field.

尽管他很累，他继续在田里工作。

Child as/though he was，he had a good command of English.

尽管他是个孩子，但他精通英语。

Hard as/though I tried to explain，he still didn't understand.

尽管我尽力向他解释，他仍然不懂。

Try as he might，he couldn't solve the problem.

尽管他很努力，但还是无法解决这个问题。