动名词类似于名词的性质,可以做主语、宾语或表语,也可以做介词的宾语；比如：Seeing is believing. I like swimming. We look forward to leaving.
现在分词与时态有关,常与be动词连用作为进行时的构成,同时现在分词还可以放在名词之后做后置定语,其实这也是省略了主语和be动词的进行时.比如：I am doing my homework. He was sleeping when I came back. The boy sitting under the tree is my brother.