* 連詞 if not 用於條件句可以用真實語氣，也可以用虛擬語氣，如：

1. 真實語氣：

If he does not study hard, he will never pass the examination.

他如果不努力學習, （將來）考試就永遠不會及格。

This baby seldom cries if he is not tired.

這個嬰兒除非在疲倦時（往往）很少哭。

If I do not understand what he says, I always ask him.

要是（平時）我不明白他所說的，我總是問他。

2. 虛擬語氣：

How could I find you here if you were not at home?

要是你不在家，我怎麼會在這裡找到你呢？（對現在虛擬）.

If he had not told me the truth about her, I would not have uderstood what she had said.

他假如沒有告訴過我她的真相，我就不會明白他所說過的話。（對過去虛擬）

* 但是，句子中只要有 but for，該句子必須用虛擬語氣。

But for your help （if you had not helped us）we should not have finished in time.

要不是你的幫助，我們就不會及時完成。

But for the rain (= If it had not rained) we should have had a pleasant journey.

要不是下雨，我們就會有一個愉快的旅途的。

1.副詞短語，意思是“不然的話, 否則” ，如：

We should do it with all our heart. If not we will leave it unfinished.

2. 連詞，引導否定的條件狀語從句。