双写的原则是“重读闭音节结尾，末尾只有一个辅音字母。第一是重读音节，第二形式上是辅元辅，辅音+元音字母+辅音字母。如put,set,stop,swim都是单音节的例子；多音节的例子有begin,forget,admit,refer,equip等，都是重读第二音节，而且形式上是辅元辅。
动词加ing作谓语的情况
这时候的动词加ing前面必须加上be动词,形成be+动词ing(现在分词)作句子谓语，构成进行时态句子。
例如:
He is reading a book now.他正在看书。
He was watching TV when his mother came in.他妈妈进来时，他正在看电视。
They've been waiting for you for three hours.他们等你三个小时了。
动词加ing做非谓语的情况
1.动词作介词的宾语要加ing(动名词)
I'm looking forward to seeing you again.我期盼着再次见到你。
The boy is so excited about hearing from his mother.男孩为收到他妈妈的来信很兴奋。
2.部分动词用动词做宾语，做宾语的动词要加ing(动名词)！
I Practising speaking English Every day.我每天练习说英语。
Could you mind opening the door?你介意开门吗？
I have finished reading three books.我已经看完三本书。
The old man Enjoys living in the country.这老人喜欢住在农村。
3.一些固定句式后面要加动词ing(现在分词)。
We have fun Playing in the park on Sunday.
Most students have problems speaking English.
4.动词修饰名词做后置定语时，要加ing(现在分成)！There is a boy Crying over there.
I felt something moving in the house.
I heard someone singing in the next room.
5.动词做主语和表语时，动词要加ing(动名词)。
Swimming is very popular in summer.
His job is teaching English.