Usage of 在 in Place complement, Time Complement In Chinese sentence, I know Place and Time complement needs to follow these orders: 

(1) Subject + Time + Place + How + Do sth

(2) Time + Subject + Place + How + Do sth

I notice that Time Complement can omit 在, for example：

（在）下课之后，玛丽去足球场。

Besides, Place Complement always starts with 在：

在包里，他拿了一本书。

So, how can I know when to use and omit 在 in Place complement, Time Complement?
when to use and omit 在 in Place complement, Time Complement?

when a sentence need to let other person fully understand or the subject is important or 
 in a sentence.

(在)包里，他拿了一本书。 在 can also omit.


It can express as 

他拿了一本(在)包里的书。


Place

我在家。


Here 在 is a important word to tell other information about where you are.