Formal:
* 请坐 (qǐng zuò): This is the most polite and formal way to say "please sit down."
* 坐下 (zuò xià): This is a less formal way to say "sit down" and is more appropriate for friends and family.
Informal:
* 坐 (zuò): This is the most casual and informal way to say "sit down."
Examples:
* 请坐,我马上就好。(qǐng zuò, wǒ mǎshàng jiù hǎo.) Please sit down, I'll be ready soon.
* 你坐下,我给你倒杯水。(nǐ zuò xià, wǒ gěi nǐ dǎo bēi shuǐ.) Sit down, I'll pour you a glass of water.
* 坐,别站着了。(zuò, bié zhànzhe le.) Sit down, don't stand.
It's important to choose the appropriate phrase based on the context and your relationship with the person you are speaking to.