Talk about locations with 在 (zài)
To talk about things being in places, use the word 在 (zài). 在 (zài) can also be said as “at”. This is actually a verb, so you don't need to use any other words to talk about something being somewhere.

Structure
[something] + 在 + [place]

Example
我 在 这里。 wǒ zài zhèlǐ. I am here.
伦敦 在 英国。 lún dūn zài yīng guó. London is in England.
兔子 在 笼子里。 tù zǐ zài lóng zǐ lǐ. The rabbit is in the cage.

Talking about negation of 在 (zài)
The negation of 在 (zài) is 不在 (bú zài)

Structure
[something] + 不在 + [place]

Example
我不在这里。 wǒ bú zài zhèlǐ. I am not here.
伦敦不在美国。 lún dūn bú zài měi guó. London is not in United State.
兔子不在笼子里。 tù zǐ bú zài lóng zǐ lǐ. The rabbit is not in the cage.

在 as the Preposition
在 (zài) also functions as a preposition. In this case, 在 indicates the location where an action occurs. Depending upon the sentence, in English, it can be mean by ‘at,’ or ‘in,’ or ‘on’.

Example
他 在 浴室里唱歌。 tā zài yù shì lǐ chàng gē. He sings in the bathroom.
弟弟 在 家吃饭。 dì dì zài jiā chī fàn. My brother eats at home.
爷爷 在 田里种菜。 yé yé zài tián lǐ zhòng cài. Grandpa grows vegetables in the field.