課本中不會教你的[U]692　unless和if…not

692　unless和if…not

① unless是從屬連詞，為書面語，用于引導條件狀語從句，意為“除非，如果……不”。unless本身具有否定意義，因此所引導的從句謂語動詞用肯定式，相當于if…not，?？蓳Q用。unless從句常位于句尾，表示強調時位于句首。

Don't come unless I call you. 我不打電話你就不要來。

=Don't come if I don't call you.

Unless you work hard，you will fail. 除非你努力工作，不然你會失敗的。

= If you don't work hard，you will fail.

② unless只能引導真實條件句，用直陳語氣，不可用虛擬語氣；而if…not既可以引導真實條件句，也可以引導非真實條件句，用虛擬語氣。因此，當if…not引導非真實條件句，謂語動詞用虛擬語氣時，就不可換用unless從句。

She will come unless it rains. 如果不下雨，她就會來。（真實條件句）

=She will come if it doesn't rain.

如果你早來十分鐘，你就會趕上火車了。

If you had come ten minutes earlier，you would have caught the train.

（非真實條件句，虛擬語氣）

Unless you had come ten minutes earlier，you would have caught the train.

③ unless如果所引導從句的謂語動詞是否定式，就不可轉換為if…not結構。

這本詞典是值得買的，除非你沒有足夠的錢。

This dictionary is worth buying unless you don't have enough money.

This dictionary is worth buying if you don't have enough money.

④ 當從句的動作或情況不發(fā)生，主句的動作或情況才可發(fā)生時，要用if…not，不可用unless。

We'll be surprised if he doesn't do it that way. 要是他不那樣做，我們倒會感到奇怪了。

⑤ unless和if…not引導的從句，要用一般現在時代替一般將來時。

除非明天下雨，不然我們將去度假。

We shall go on a holiday unless it rains tomorrow.

We shall go on a holiday if it doesn't rain tomorrow.

⑥ unless如果所引導從句的主語和主句的主語相同，謂語中又有be動詞時，從句的主語和be動詞可省去。

She won't come unless invited. 如果不邀請，她不會來。（省去she is）