The 在 (zài) character has several different uses, and since it's a character that will appear soon after you begin learning Chinese, it's a good idea to get yourself accustomed to its different functions. Most of them have to do with location, as primarily, 在 (zài) is used to express the phrase 'to be located…'
Here are some rules and sentence patterns to follow when using the 在 (zài) character and also some advice on how not to use it:
1. Expressing location with 在 (zài)
The 在 (zài) character can be used to indicate location and often be used to state 'to be located at'.
subject + 在 + place
他在家。(tā zài jiā) He is at home.
The negation for 在 (zài) is 不在 (bú zài) :
他不在家。(tā bú zài jiā) He is not at home.
在 (zài) also functions as a preposition. It indicates locations where an action occurs. It is often used as 'at', 'in', 'on'.
2. Indicating location with 在 (zài) before verbs
If you need to include the place where an action takes place, you can use 在 (zài).
To indicate the location that a verb takes place in, 在 (zài), followed by a location, comes before the verb.
Subj. + 在 + Place + Verb + Obj.
There are some verbs that do not follow the pattern above. These verbs include 住 (zhù), 放 (fàng), 坐 (zuò) and 站 (zhàn) and come after the verb.
Subj. + [住/放/坐/站] + 在 + Location
你住在深圳吗？(nǐ zhù zài shēn zhèn ma?) Do you live in Shenzhen?
3. Using 在 (zài) and 正在 (zhèng zài) to suggest ongoing actions in the present
在 in action
在 (zài) and 正在 (zhèng zài) can only be used with actions, not adjectival, stative or modal verbs.
哥哥在打球。(gē ge zài dǎ qiú) The older brother is playing with a ball.
我在看书，妈妈在洗碗。(wǒ zài kàn shū, mā ma zài xǐ wǎn) I am reading a book and mom is washing dishes.
4. Using 现在 (xiàn zài) in the present
现在 (xiàn zài) can be used when talking about actions occurring in the present.
汽油现在贵了。√ (qì yóu xiàn zài guì le)
Oil is expensive now.
5. Using 在 (zài) for decisions
在 (zài) can sometimes be used to express an abstract idea. In this situation, 在 (zài) is used to mean 'up to you'.
去不去在你。(qù bù qù zài nǐ) It is up to you if you go or not.
6. 在 (zài) to express the scope of something
zai to indicate scope
In this case, 在 (zài) is combined with 方面 (fāng miàn) to make the phrase 'in the area of…' or 'on the topic of…'
在 + subject/area + 方面 + comment
在音乐方面，他是个天才。(zài yīn yuè fāng miàn, tā shì gè tiān cái)
He is very talented musically.
7. 在 (zài) used to express existence and belief
妈妈对我们的关心无处不在。(mā ma duì wǒ men de guān xīn wú chù bú zài.)
Mother was concerned about us in every way.
Source: Written Chinese