連接詞主要有 if, unless, as/so long as, on condition that 等。.
    if 引導(dǎo)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。可轉(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.