He was doing his homework when his mother came in.
他妈妈进来时，他正在做家庭作业。
1、动词作介词的宾语要加ing(动名词)
The girl is so excited about hearing from her boyfriend.
这个女孩收到男朋友的来信很兴奋。
I'm looking forward to seeing you again.
我期盼着再次见到你。
2、部分动词用动词作宾语。作宾语的动词要加ing(动名词)
The student was practising making a new vowel sound.
这个学生正在练习发一个新的元音。
I don't mind doing chores.
我不介意做家务。
Mary enjoys reading very much.
玛丽非常喜欢阅读。
3、一些固定句式后面要加动词ing(现在分词)
We're going to have fun doing sports after school.
放学以后我们将愉快地进行体育活动。
He is quite used to working hard.
他很习惯努力工作。
Most students have problems speaking English.
大多数学生说英语有困难。
I prefer walking to cycling.
我愿意步行，而不愿意骑自行车。
