以前 and 以后
以前 and 以后 are used to indicate that an action happened in the past or will happen in the future. They can be either adverbs or postpositions.
The postpositions 以前 and 以后
以前 
indicates that an action happened or will happen before until a certain moment or event.
以后 indicates that an action happened or will happen after a certain moment or event.
In a sentence, 以前 and 以后 are used as parts of an adverbial modifier of time. A subject is placed either at the beginning of a sentence or between 以前/以后 and a predicate.
indicates that an action happened or will happen before until a certain moment or event.
以后 indicates that an action happened or will happen after a certain moment or event.
In a sentence, 以前 and 以后 are used as parts of an adverbial modifier of time. A subject is placed either at the beginning of a sentence or between 以前/以后 and a predicate.Scheme
subject
time/action
以前/以后
(subject)
predicate
Pay attention
以前 can be shortened to 前, and 以后 can be shortened to 后.
Examples
我们天黑以前应该能赶到
We should be there before dark
wǒmen tiān hēi yǐqián yīnggāi néng gǎndào
学生须在八点前到校
Students must be at school by 8 a.m.
xuéshēng xū zài bā diǎn qián dào xiào
遇到了她以后，我的生活就不一样了
My life has changed since I met her
yùdào le tā yǐhòu, wǒ de shēnghuó jiù bù yíyàng le
明天下午六点后图书不可以借出
Books cannot be borrowed after 6 p.m. tomorrow
xiàbān hòu, wǒ yào xuéxí zhōngwén
The adverbs 以前 and 以后
以前 and 以后 can also be adverbs and used independently.
In a sentence, they are also used as parts of an adverbial modifier of time.
and 以后 can also be adverbs and used independently.
In a sentence, they are also used as parts of an adverbial modifier of time. Pay attention
In this case, 以前 and 以后 cannot be shortened.
Examples
以前我不喜欢吃蔬菜
I didn't like vegetables before
yǐqián wǒ bù xǐhuan chī shūcài
这个房子以前没有电
There was no electricity in this house before
zhè ge fángzǐ yǐqián méi yǒu diàn
我以后也许会对此很后悔
Maybe I'll regret it
wǒ yǐhòu yěxǔ huì duìcǐ hěn hòuhuǐ
以后，你不要再迟到了
Don't be late anymore
yǐhòu, nǐ bú yào zài chídào le