The use of 在(zai)
Location expressions include three parts : 在 + place + localizer.
Location expressions are used in the following positions:

㈠ Before the verb, location expressions are used as a prepositional phrase to indicate location. Some of the most common localizers are
上(shàng) above; on
里(lǐ) inside
外(wài) outside
前面(qián) (miàn) in front of
后面(hòu) (miàn) behind
中间(zhōng) (jiān) between; in the middle of
旁边(páng) (biān) near, by side of, beside, next to
Sentence pattern: Subj. + 在 + place + Loc. + V

Example
1, The teacher is writing on the blackboard.
老(lǎo) 师(shī) 在(zài) 黑(hēi) 板(bǎn) 上(shàng) 写(xiě) 字(zì) 。
2, Children are playing in the park.
孩(hái) 子(zi) 们(men) 在(zài) 公(gōng) 园(yuán) 里(lǐ) 玩(wán) 。
3, Mr. Li is speaking in front of the classroom.
李(lǐ) 小(xiǎo) 姐(jiě) 在(zài) 教(jiào) 室(shì) 前(qián) 面(miàn) 讲(jiǎng) 话(huà) 。

㈡ Location expressions can also be placed after the verb to indicate a particular place.
Sentence pattern: Subj. + V + 在 + place + Loc.
Example
1, My family lives outside of town.
我(wǒ) 家(jiā) 住(zhù) 在(zài) 城(chéng) 外(wài) 。
2, The doctor is standing next to the nurse.
医(yī) 生(shēng) 站(zhàn) 在(zài) 护(hù) 士(shì) 旁(páng) 边(biān) 。
3, My mother is sitting on the chair.
我(wǒ) 妈(mā) 妈(mɑ) 坐(zuò) 在(zài) 椅(yǐ) 子(zi) 上(shàng) 。