if的条件状语从句的三种用法

方法/步骤

1、主将从现（主句为一般将来时,从句为一般现在时）； eg:If it rains(从句) ,I will stay at home（主句）. =I will stay at home if it rains.

2、主句含有情态动词（must、may、can etc.）从句为一般现在时； eg:If you drive too fast ,you may have an accident. =You may have an accident if you drive too fast

3、主句为祈使句,从句要用一般现在时态； eg:Don't play computer games if you don't finish your homework.

备注： 1、If 于句首时中间用逗号隔开,主句后不用逗号. 2、if 可用unless(除非...) 替换.unless= if...not... 3、注意虚拟语气主句和从句都为一般过去时

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