英语中一些行为动词后有加doing 又有加to do，怎么回事啊

有区别。

接doing（动名词）的常见动词：avoid , admit , advise , consider , delay , enjoy , finish , forbid , give up , imagine , keep , mind , miss , practise , permit , resist , risk , suggest , stop 等

接to do（不定式）的常见动词：decide，expect，refuse，wish，hope，order，promise，pretend，offer，happen，seem，make up one′s mind，used，be about，be able等

有些动词(stop,forget,remember,regret,like等)后接不定式或动名词时，意思差别很大。
I must remember to post the letter.我必须记着要发信。(信还未寄出)?

I remember posting the letter.我记得发了信了。(信已寄出)

I like swimming.(喜欢游泳这项运动)

I like to swim this Friday.(喜欢在星期五去游泳这一具体动作)

一贯行为to do经常做doing