連接詞主要有 if， unless， as/so long as， on condition that 等。if 引導的條件句有真實條件句和非真實條件句兩種。非真實條件句已在虛擬語氣中闡述。

　　unless = if not.　 例如：

　　Let's go out for a walk unless you are too tired. 如果不太累，我們去散散步。

　　If you are not too tied， let's go out for a walk.

　　典型例題

　　You will be late ___ you leave immediately.

　　A. unless　 B. until　 C. if　　D. or

　　答案A.句意：除非你立即走，否則你就回遲到的。可轉化為 If you dong leave immediately， you will be late.B、D句意不對，or表轉折，句子如為 You leave immediately or you will be late.