有些英语单词后面加个 in' 也有加 'in 的,这是什么意思?

是口语形式~
比如ain't
为am not,is not,are not,has not,have not的口语形式~
例：If it ain't broke,don't fix it.没有坏的话就不要去修理它（不必自寻烦恼）.
Means that if something is in a satisfactory state,there is no reason to try and change it