but I (could) not understand him

问题描述:

but I (could) not understand him
but I could not understand him.
but I can not understand him.
but I don't understand him.
有何区别?是否可以互换?
整句意思有何不同吗?

这三句意思是差不多的,如果没有时态要求是可以互换的
唯一的区别就是could 和can的时态,could 表示过去时,can是现在时