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.　
　 let's go out for a walk unless you are too tired.
　 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�？赊D(zhuǎn)化為 if you dong leave immediately, you will be late.　b、d句意不對，or表轉(zhuǎn)折，句子如為 you leave immediately or you will be late.