The preposition 在

In Chinese, 在 (zài) combines with a place word or phrase to say where an action happens.

This phrase normally stands before the verb.

Basic structure

Subject + 在 + place + verb

Meaning: someone does something in some place.

我(Wǒ)在(zài)学(xué)校(xiào)吃(chī)午(wǔ)饭(fàn)。

I have lunch at school.

我 + 在学校 + 吃午饭 I + at school + have lunch

我(wǒ)I

在(zài)学(xué)校(xiào)at school

吃(chī)午(wǔ)饭(fàn)have lunch

Here 在学校 states where the action happens.

他(Tā)爸(bà)爸(ba)在(zài)医(yī)院(yuàn)工(gōng)作(zuò)。

His father works at a hospital.

他爸爸 + 在医院 + 工作 his father + at the hospital + work

他(tā)爸(bà)爸(ba)his father

在(zài)医(yī)院(yuàn)at the hospital

工(gōng)作(zuò)work

Here 在医院 names where the working happens.

你(Nǐ)在(zài)哪儿(nǎr)买(mǎi)菜(cài)？

Where do you buy groceries?

你 + 在哪儿 + 买菜？ you + where + buy groceries?

你(nǐ)you

在(zài)哪儿(nǎr)where

买(mǎi)菜(cài)buy groceries

Here 在哪儿 asks where the action happens.

在 + place + verb

Meaning: it states where the action happens.

Examples:

在(zài)学(xué)校(xiào)吃(chī)午(wǔ)饭(fàn)

have lunch at school

在(zài)医(yī)院(yuàn)工(gōng)作(zuò)

work at a hospital

在(zài)哪儿(nǎr)买(mǎi)菜(cài)？

where do you buy groceries?