怎样区分祈使句和动词不定式作主语,也就是说填空时是填ing形势还是原形

祈使句是动词原形开头,比如我们常见的Open the door!/ Close the door!
动词不定时作主语就是动词前面有to,比如To finish the work before five is really hard for me.
而这里你所说的ing,是动名词作主语,比如Smoking is prohibited here.等等.