Catch up 可以表示追上前面的人，或补上落下的学习、工作；catch up on sleep 则是补觉。它还可以表示隔了一段时间后见面、交流近况：Let's meet and catch up。
想在对话中用 catch up 吗？
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catch up with + person; catch up on + work/sleep/news; catch up over + coffee/dinner
- I missed two weeks of classes, so I need to catch up on the reading.
- We hadn't seen each other since university, so we met for dinner to catch up.
- Walk a bit slower so that your little brother can catch up with us.
修改原因
修改前I was ill for a week, so now I have to catch up the lessons I missed.
修改后I was ill for a week, so now I have to catch up on the lessons I missed.
这里容易漏掉介词。Catch up 后面说明补上什么内容时，要用 on：catch up on the lessons、catch up on sleep。
Catch up 表示已经落后，现在正缩小差距。Keep up with 则是始终跟上节奏，不先掉队：It's hard to keep up with the new material。如果已经落下，就需要 catch up。
动手练习
Alex is already far ahead of us. Let's walk faster to ___ him.
查看答案
catch up with — 目标是追上一个人，因此用 with：catch up with him。On 后面接落下的工作、睡眠等内容，不接这里要追赶的人。
想一件最近错过或落下的事，例如课程、工作任务或新闻。说明具体需要补上什么，以及什么时候完成。
I missed three French lessons last week, so I'll catch up on the grammar exercises this Sunday.
练习把新词用准确
用正在学的词造句。LetsWords 检查用法和语法，提供修改与母语解释，再通过复习巩固。