Talk about the past and the future in general
Both 以前 (yǐqián) and 以后 (yǐhòu) work the same way. They are used to talk about something that has happened at an unspecified time in the past or that will be happening at an unspecified time in the future. Let’s start with 以前 (yǐqián) to talk about the past.
以前, before
Structure
[以前+S+V+O]
[S+以前+V+O]
[S+以前+V+O]
Examples
以前我 住 在 北京。Before, I lived in Beijing.
我以前住 在 北京。I lived in Beijing before.
我以前住 在 北京。I lived in Beijing before.
以前他 学 中文。Before, he learned Chinese.
他以前学 中文。He’s learned Chinese before.
他以前学 中文。He’s learned Chinese before.
以前她 说 汉语。Before, she spoke Chinese.
她以前说 汉语。She spoke Chinese before.
她以前说 汉语。She spoke Chinese before.
以后 to talk about the future in general
以后 is used to describe things that will happen at an unspecified time in the future, and, like 以前, can be used to talk about both very close upcoming events and events far away in the future. 以后 here can be translated as “in the future”, or “later”.
Structure
[以后， S+ V + O]
[S+以后+V+O]
[S+以后+V+O]
Just like 以前, 以后 can be placed indifferently before or after the subject when you’re talking about time in general. It is common to add a comma after 以后 if it’s placed at the beginning of the sentence.
Examples
以后，他 要 学 中文。Later, he’ll learn Chinese.
他以后要 学 中文。He’ ll learn Chinese later.
他以后要 学 中文。He’ ll learn Chinese later.
以后，我 要 去 北京 上学。In the future, I’ll go study in Beijing.
我以后要 去 北京 上学。I’ll go study in Beijing in the future.
我以后要 去 北京 上学。I’ll go study in Beijing in the future.
以后，很多 人 都 会 去 中国。In the future, many people will go to China.
很多人以后都 会 去 中国。Many people will go to China in the future.
很多人以后都 会 去 中国。Many people will go to China in the future.
Before a specific action/event/time
If you want to talk about something that happened in the past or in the future at a more specific moment, or to talk about a specific event, you can also use 以前 and 以后. You can put an event, a specific time or an action before 以前 or以后.
Structure
[action/event/time+ 以前/以后， S+V+O]
This pattern is close to “before/after something, something happened” in English. Unlike when you're talking about the past or the future in general, when you're talking about a specific event, action or time, 以前 and 以后 must always be placed after and cannot be placed before.
Examples
回家以前，我 吃 了 苹果。I ate an apple before I went home.
睡觉以前，我 刷 了 牙。Before I went to sleep, I brushed my teeth.
十点以前，我 在 家。I was at home before 10 o’clock.
吃饭以后，我们 去 看 电影。We’re going to go see a movie after we eat.
去 中国 以后，他 学会 了 写 汉字。He learned how to write Chinese characters after he went to China.
两天以后，我 去 北京。I’ll go to Beijing in 2 days.
下午两点以后，我 去 医院。I’ m going to the hospital after 2 p.m.