Differences between Chinese characters “当” and “在”

When it is used to indicate time, "当(dāng)" can't followed by a single temporal noun, while "在(zài)" can. Example:

表示时间时，“当”后面不能跟单个时间名词，“在”可以。例如：

在那时，我们都很年轻，还不懂得什么是爱情。

zài nà shí, wǒ men dōu hěn nián qīng, hái bǔ dǒng dé shén me shì ài qíng.

At that time, we were both young and didn't know what love meant to us.

At that time, he had gone.

(WRONG) 当那时，他已经走了。

"当(dāng)" can only mean a time point rather than a time period. "在(zài)" can either mean a time point or a time period. Example:

“当”只能表示时间点，不能表示时间段；“在”既可以表示时间点，也可以表示时间段。例如：

当我进房间的时候，他正在修暖气。

dāng wǒ jìn fáng jiān de shí hòu, tā zhèng zài xiū nuǎn qì.

When I entered the room, he was fixing the heating installation.

We have met each other since several years ago.

(WRONG) 当几年前，我们就见过面。

我在云南时去西双版纳旅游过。

wǒ zài yún nán shí qù xī shuāng bǎn nà lǚ yóu guò.

I have visited Xishuangbanna during my trip to Yunnan.

这座城市在十几年前就通地铁了。

zhè zuò chéng shì zài shí jǐ nián qián jiù tōng dì tiě le.

This city has launched subway services since a decade years ago.

When it is used to indicate locations or places, "当(dāng)" can be used with locative words or positional nouns. It can only be used with a few nouns. "在(zài)" can be used with locative words or positional nouns. For example:

表示处所时，“当”不能与处所词、方位词组合，只能与少数名词组合；“在”可以与处所词、方位词组合。例如：

请不要在地上乱扔烟头。

qǐng bú yào zài dì shàng luàn rēng yān tóu.

Please do not throw cigarette butts on the ground.

他当着大家的面，突然跪下向女友求婚。

tā dāng zhe dà jiā de miàn, tū rán guì xià xiàng nǚ yǒu qiú hūn.

He suddenly knelt down and proposed to his girlfriend in front of everyone.