为什么英语里有的单词后加ing例如 fly+flying

这个要靠背的,通常英文字后面有e,e都会不见
英语动词+ing 是进行时
如果是动词原型+ing 那就是现在进行时 那个的句式是：be +动词ing 表示正在发生：I'm falling love with him.我爱上他了.
如果是动词过去式+ing 那就是过去进行时 这个很少见 表示过去正在发生
ing又分为动名词和动词的现在分词两种.在现代语法中不再具体强调其分别的,统一的叫做动词的ing形式.
ing形式可以作句子的主语
如 Smoking is a bad habit.
ing做宾语
I like sleeping.
ing做定语（多含有主动、正在进行、使得...的含义）
The sleeping baby is my son.
The exciting news made them break into tears.
ing做宾补
The result of the test made the student crying.
还有好多,建议你可以查下语法书关于非谓语动词和动名词的部分.
1.正在进行时中,谓语动词后加ing.如:I'm playing football.
2.当动词做主语时,要用动名词,即动词加ing.如:
Making salad.
3.一些特殊表达.动词后再用动词要用动名词.如:enjoy doing sth.practice doing sth.mind doing sth.finish doing sth.keep doing sth.等
下列结构含有ing:
1.keep doing
2.keep /carry on doing
3.keep sb.Doing
4.enjoy doing
5.finish doing
6.be afraid of doing
7.be worth doing
8.be busy doing
9.how about doing//what about doing
10.spend some time (in)doing
11.spend some money (in) buying
12.feel like doing
13.stop/keep/prevent … from doing
14.thank you for doing
15.thanks for doing
16.do some cooking/cleaning/reading/shopping/washing
17.go swimming/fishing/shopping/skating/boating
18.mind/practise doing
19.prefer doing … to doing…
20.can’t help doing
21.there is/are +n.+ doing …