详细的动词-ing的用法?以下的句子为什么用动词-ing?

(1)He thinks that watching English movies isn't a bad way to learn a language.(2)He finds watching movies frustrating beacause the people speak too quickly.(3)He said that memorizing the words of pop songs also helped a little.

be动词are am is 后面的动词要使用ing形式。
如：
she is reading a book.
动词做主语，也要加ing如：
watching too much TV is bad for our health

把动词watch变成名词watching,所以加ing

感官动词如SEE，THINK，FIND等后的从句要用-ING形式。

因为动词需要变成名词 才能做介词的宾语   动词加ing 就是动名词 可以起名词作用。

任何介词后面都是如此。

用来形容名词，这句话里就是动名词做主语成分，一是动名词就句子本身而言，是宾语从句，可以切分为 1.he thinks(主句）think是谓语动词 2.watching English movies isn't a bad way to learn a language（主句所引导的从句）isn',有两种;t 是系动词 而watching虽然是动词，一般动词加ing，二是作分词