我不太明白说英语

I am not very clear about that.

I have no exact idea. 相信我，我英语专业的

I do not quit understand.

Sorry, I can just understand a little about that

I don't understand what he said. ~

I can't understand at all

如果是在正常跟外国人讲话，听不太清楚就说Sorry?别人就会再解释或者重复一遍的。这是最普遍的。如果硬要翻译就I can't follow/catch you 。不可能一个英文对应一个中文，关键是要看别人的环境与风格。

I can't understand what he said

“我非常不明白.”用英语 I don't understand at all. 希望可以帮到你，如果满意请采纳，谢谢。