问什么情况下动词加ing，什么情况下用动词不定式，这个取决于它前面用的动词，不取决于这个动词本身，如跟在enjoy后的动词要加ing;跟在want后要用动词不定式。综合讲，动词在不做谓语的时候就要用它的非谓语形式，即-ing，-ing，-ed和to do形式，至于什么情况下用-ing形式，那就要搞清楚这四种非谓语结构的区别了。具体来看下文：

　　我们先来看作谓语的情况.

　　这时候的动词加ing前面需要加上be动词,形成be+动词ing，作句子谓语，构成进行时态句子。

　　例如:He is reading a book now.他正在看书.

　　He was watching TV when his mother came in.他妈妈进来时，他正在看电视。

　　They've been waiting for you for three hours.他们等你三个小时了.

　　现在我们来看动词加ing做非谓语的情况。

　　1.动词作介词的宾语要加ing。

　　I'm looking forward to seeing you again.我期盼着再次见到你.

　　The boy is so excited about hearing from his mother.男孩为收到他妈妈的来信很兴奋。

　　2.部分动词用动词做宾语。做宾语的动词要加ing!

　　I Practising speaking English Every day.我每天练习说英语.

　　Could you mind opening the door?你介意开门吗?

　　I have finished reading three books.我已经看完三本书.

　　The old man Enjoys living in the country.这老人喜欢住在农村.

　　3.一些固定句式后面要加动词ing。

　　We have fun Playing in the park on Sunday.

　　Most students have problems speaking English.

　　4、动词修饰名词做后置定语时，要加ing!

　　There is a boy Crying over there.

　　I felt something moving in the house.

　　I heard someone singing in the next room.

　　5.动词做主语和表语时，动词要加ing。

　　Swimming is very popular in summer.

　　His job is teaching English.

　　综上所述，动词在以上六种情况需要加ing!