在 (zài) — at, in, on
在 zài is one of Mandarin's most useful words. As a verb on its own, it means 'to be at / be in / be located'. As a preposition before another verb, it means 'at' or 'in' a place. The pattern is the same either way: subject + 在 + place. 我在家。 = I'm at home. 她在学校。 = She's at school. 我们在中国。 = We're in China. There's no separate verb 'to be located' and no 'is' to add — 在 is the whole predicate. The matching here/there words are 这里 / 这儿 and 那里 / 那儿.
在 as a standalone verb
When 在 is the main verb, the sentence is just subject + 在 + place. 我在家。 = I'm at home. 爸爸在学校。 = Dad's at school. 猫在桌子上。 = The cat is on the table. NO 是 — never 我是在家. 在 already carries the 'be' meaning. To negate, use 不: 我不在家。 = I'm not at home. (Don't confuse this with 没有, which negates 有 specifically — see lesson 9.)
Asking where: 哪里 / 哪儿
哪里 nǎlǐ or 哪儿 nǎr = where. (Northern speakers — and Beijing — prefer 哪儿; Taiwan and most textbooks use 哪里. Both are fine.) Question pattern, subject + 在 + 哪里?: 你在哪里? = Where are you? 妈妈在哪儿? = Where's mom? Reply with the place: 我在家。 她在学校。 Don't confuse 哪 (which/where, 3rd tone) with 那 (that, 4th tone) — same character family, different word.
Here and there
这里 zhèlǐ / 这儿 zhèr = here. 那里 nàlǐ / 那儿 nàr = there. Same north/south split as above — pick one and be consistent. Use them like any place: 他在这里。 = He's here. 书在那儿。 = The book's there. 你的车在哪里?——在那里。 = Where's your car? — Over there.
在 + place + verb (the preposition use)
When 在 modifies another verb, the structure is subject + 在 + place + verb + (object). The 在-phrase says WHERE the action happens, and it ALWAYS comes before the verb — never after. 我在家看书。 = I read books at home. 她在学校学习。 = She studies at school. 我们在中国吃饭。 = We eat (meals) in China. Compare to English, which puts 'at home' after the verb: this is one of the most common English-speaker mistakes. Wrong: 我看书在家; right: 我在家看书.
Place words: 上, 里 — locating things
Most concrete locations attach a position word at the end. 桌子上 = on the table (table-on). 家里 = inside the home / at home (home-inside). 学校里 = inside the school. So 猫在桌子上。 = The cat is on the table. 妈妈在家里。 = Mom's inside the house. With proper places (学校, 中国, 北京) you can drop the position word: 她在中国。, not 她在中国里. With everyday objects (桌子, 椅子), you almost always need 上 or 里.
