当要用动词相关的动作作主语时,动词要用+ing形式.
Smokingisbadforyourhealth.
用动词原形的几情况
1.加助动词do,does或did的肯定句变成否定句时,其后面的动词要用动词原形;
Hedidn’tworkhardatEnglishlastterm.
2.情态动词后面要用动词原形;
MustIbeathometonight?
3.begoingto,will,shall等构成的将来时态,其后面的动词要用动词原形;
Therewillbeonanewinterestingfilmatthecinema.
4.祈使句要用动词原形;Becareful,Lintao!
5.let开头的祈使句,其后面的动词要用动词原形;
Let’shavealookatthepictures.
6.hadbetter后面要用动词原形;
you’dbetterputonmoreclothes.It’sverycoldoutside.
7.whynot之后要用动词原形;
Whynotcomealittleearlier?=Whydon’tyoucomealittleearlier?
8.动词不定式to之后要用动词原形;
Pleasegivemesomethingnicetoeat.
9.feel,hear,see,watch等感官动词之后要用动词原形.如果表示这一动作正在进行,就用现在分词.
Sometimeswecanhearagirlsingintheroom.
Ifeelmyleftleghurtingbadly.
那些都是我自己总结的,仅你参考.