Question

if从句的三种用法

Answer 1

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式
eg:If I was a bird ,I could fly.（事实我不可能是鸟）