什么样的动词在后面加ing时要双写最后一个字母，比如游泳swim-swimming

1、以重读闭音节结尾,末尾只有一个辅音字母的动词,要双写这一辅音字母,再加-ing.例如：run-running, swim-swimming等.
2、以-al,-er,-ill结尾的动词,即使重音不在最后一个音节上,也要双写,再加-ing 如：travelling等.
可以加动词ing的
①及物动词.She likes drawing very much.；
②某些短语动词. Mary is thinking of going back to New York.；
③ do+限定词（my, some, any, the等）+ -ing,表示“做…事”之意,如：We often do our cleaning on Saturday afternoon. Will you do any shopping on Saturday this afternoon?
④介词：Her sister is good at learning physics.；
⑤形容词worth, busy等：This book is well worth reading. –ing作宾语带有宾语补足语时,要用it作为形式宾语,而将作宾语的-ing后置,如：We found it no good talking like that. Do you think it necessary trying again?

来自 http://zuoye.baidu.com/question/305042a63d2b04c5cc601fce057f066a.html

倒数第二个字母是元音字母，倒数第三个是辅音的单词一般都需要双写，然后记几个不双写的特例就能解决这个问题了

stop,swim,shop,sit,run

以a，e，i，o，u加其他21个字母结尾的

sit