What is the difference between yǐqián (以前) and zhīqián (之前)? I'd like to have some way to differentiate between yǐqián (以前) and zhīqián (之前), in order to help with memorising them both.  Referring to Modern Mandarin Grammar (Claudia Ross, Routledge 2006), it seems to suggest that the only point of differentiation is that zhīqián is  more literary.  Is this the only difference?  If not, what do I need to know?
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 &quot;earlier&quot; while yiqian is similar to &quot;used to&quot;. For example:

我以前抽烟,但我现在不抽了. &quot;I used to smoke, but I quit now.&quot;
我之前刚抽了一支烟. &quot;I just smoked one cigarette earlier.&quot;

In its meaning of of &quot;used to&quot;, 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. &quot;At school, we used to do a lot of multiplication-table drills.&quot;
× 之前 我们上学得反复背诵乘法表.


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。