I can’t fully understand what you mean

I can't fully understand what you mean
我不能完全明白你的意思,我翻译成I can't fully understand what you mean,can't fully understand 的表达是否正确,我看别人都是这么翻译的：I could not fully understand what you mean 和 I do not fully understand what you mean ,我是自学英语的好多知识都不懂.

可以这么用.
can表示ability,这么说表示你听不懂对方讲什么,比如你和一个老外对话,你可以这么讲.
如果用do,表示你ability没有问题,但是可能是别的什么原因导致未听清,示意对方再重复一边.
关于could,最好不要这么用,原因如下：
In conversation.they usually express doubt,with could showing the greatest degree of uncertainty or tentativeness. －Longman Spoken & Written English
比如 That could be her.表示”我觉得也会是她“