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,也可以做前置定语,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.
其实,现在分词可以做很多成分的,不只是定语,还有状语,补语,表语等等,很复杂的啦.
如果不是很需要的话,就不要那么纠结这些问题了.