为什么有的词后面要加ing

语法需要,如：
固定结构,enjoy doing sth,look forward to doing sth;
时态结构：进行时：Look,the boying is playing with his mum.I was cooking when you called yesterday evening.
动词加ing作形容词：That was really exciting news.He's an interesting man.
动词作主语时,须变成加ing的动名词：Walking is good exercise.Playing a kite is fun.

英语中现在进行时态的动词后边要加 ing，例如 I am making coffee 我正在磨咖啡。
但是也有一些词本身就有这样的字母组合，比如 thing， sing，等等。

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