...做主语,什么时候是动词原形做主语,什么时候是动词ing做主语...答:2、常用不定式做主语的句型有:(1)It’s difficult (important, necessary) for sb. to do (2)It’s kind (good, friendly, polite, careless, rude, cruel, clever, foolish, brave) of sb. to do.3、常用动名词做主语的句型有:It’s no good (use, fun) doing.It’s (a) waste...