Sorry,I can't follow you.Please say it again.当中的I can't follow you 用I don't understand.还是用I can't catch up with you .用哪个好.

Sorry,I can't follow you.Please say it again.当中的I can't follow you 用I don't understand.还是用
I can't catch up with you .用哪个好.

用I can't catch up with you比较好,因为前面的句子是I can't follow you,句子是表达我不能跟上你,请再说一遍,而catch up with这个短语也就是跟上,赶上的意思,而 don't understand是不能理解的意思,跟句子的意思不符

用第二个好啊，用第二个显示出你英语的修养，这两个都有我跟不上我不理解的意思，谢谢采纳…