What is the difference between yǐqián (以前) and zhīqián (之前)?

As a native speaker，I think “之前” and “以前” is similar ,but “以前” is more common，more general。 “之前” used a little，especially in oral talking。

just some cases：

以前
english used as Before or ago，and usually used to start a sentence

eg：1、很久以前，有一座山 long time ago,there is a mountion over there.

2、以前，我有一个洋娃娃 I used to have a doll。

之前

usually used behind some preposition or sentence or subject

1.我希望我们能在最后期限之前完成这项工作。

1.I hope we can finish this before the deadline

in conclusion：

以前

1.usually the present or a certain period of time earlier

2.Refers to the past

1.比现在或某一时间早的时期。

2.泛指从前，以往。

之前

in front of a time or place. Generally used in time

表示在某个时间或处所的前面。一般多指时间

index:

0、chinese wiki grammar

1、在线汉语字典

2、之前 英文

3、新闻

4、百度百科 之前

5、百度百科 以前

从前 (cónɡqián) always refers to a time far in the past, while 以前 (yǐqián) is closer to the present.

以前 (yǐqián) can be used after time words while 从前 (cónɡqián) cannot.

Examples:

Tiānliàng yǐqián, wǒ bìxū qǐchuáng.

天亮 以前，我 必须 起床。

Before dawn, I must get up.

Tā shísuì yǐqián zhù zài Běijīng.

他 十岁 以前 住 在 北京

Before he was ten years old, he lived in Beijing.

以前 (yǐqián) refers generally to a time before now, and it can be placed at the beginning or the middle of a sentence.

Examples:

Yǐqián, wǒ zài xiǎoxué jiāoshū.

以前， 我 在 小学 教书。

I used to teach at primary school.

从前 (cónɡqián) is always used at the very beginning of a story. It is similar to the English phrase "a long time ago," or “once upon a time.”

Example:

Cóngqián, sēnlín lǐ zhùzhe yíwèi lièrén.

从前， 森林 里 住着 一位 猎人。

A long time ago, there lived a hunter in the forest.

Source from eChineselearning.com

我只能简单地说一说。

“之前”是介词，“以前”是副词也是介词。

在你不能断定该用“以前”还是“之前”地方，写上“过去”。能用“过去”的地方，就只能用“以前”，不能用“之前”。不能用“过去”的地方，就可以用“之前”也可以用“以前”。

请看下面的例句：

对：以前，中国骑自行车的人很多。（可以改为“过去，中国骑自行车的人很多。”） 错：之前，中国起自行车的人很多。

对：在出现汽车之前人们骑马。 对：在出现汽车以前人们骑马。（不能改为“在出现汽车过去人们骑马。”）

很遗憾，越来越多的说中文的人把“之前”当作“以前”用。It is very bad more and more Chinese people are using “之前” where they should use “以前”.

I'm not a native speaker, but it seems to me that in some cases the usage of these two terms is different and not interchangeable.

For example, I think you could say 我三岁之前，我只喝牛奶 (or 只能喝牛奶), to have the meaning "Before I was three", but it would sound a little odd to say 我三岁以前，我只能喝牛奶, wouldn't it?

I think both terms can mean "in the past", but it seems to me that if you are referring to an event that occurred prior to some other event in the past, 之前 sounds better (but for something that occurs before something else in the future, both 之前 and 以前 are OK - for example, 我去欧洲之前，我需要学习法语 or 我去欧洲以前，我需要学习法语 (although personally I still like the sound of 之前 better than 以前 in this sentence too).

Please correct me if I'm wrong.

As a native Chinese speaker, I agree with what you found in the book. Two words are almost identical except that Zhiqian is more literary. But, on a second thought, it seems that zhiqian is similar to "earlier" while yiqian is similar to "used to". For example:

我以前抽烟,但我现在不抽了. "I used to smoke, but I quit now."

我之前刚抽了一支烟. "I just smoked one cigarette earlier."

In its meaning of of "used to", 以前 can be used to start a sentence, while 之前 cannot:

以前我们上学得反复背诵乘法表. "At school, we used to do a lot of multiplication-table drills."

× 之前 我们上学得反复背诵乘法表.

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以前：

1. 在时间上，比现在，或某一时间（或某一事件）早的时期。如： 三岁以前，我只能喝牛奶。
2. 泛指从前，以往。如：我以前抽烟，但我现在不抽了。

之前：

1. 表示在某个时间或处所的前面。如： 他在满17岁之前发表了第一篇小说。
2. 在事情发生的先后次序方面，指一事先于另一事发生。如：吃饭之前要洗手。

一般说来，“之前”所指的时间事件或处所，与所参照的时间事件或处所相隔不远。“以前”就比较宽泛。如：“吃饭之前要洗手”意思是说，洗手后很快吃饭。但“吃饭以前要洗手”则没有强调洗手后很快吃饭。早晨洗了手后，吃晚饭之前不洗手，也算是“吃饭以前洗手了”。

另外，“之前”要出现在某事某时某处之后，因为“之前”中的“之”指代前面出现的某事某时某处。所以“之前”用在句子开头是错的，因为前面没有所指的东西。