Formal:
* 请坐 (qǐng zuò): This is the most polite and formal way to say "sit down". It literally means "please sit".
* 坐下 (zuò xià): This is also formal, but less polite than "请坐". It means "sit down".
Informal:
* 坐 (zuò): This is the most common and informal way to say "sit down". It simply means "sit".
* 坐下来 (zuò xià lái): This is slightly more formal than "坐" and is often used when asking someone to sit down on the floor.
Example:
* 请坐,请喝茶。(Qǐng zuò, qǐng hē chá.) Please sit down and have some tea. (Formal)
* 坐下,我们聊聊。(Zuò xià, wǒmen liáo liáo.) Sit down, let's talk. (Informal)
Note: The choice of which phrase to use will depend on the situation and your relationship with the person you're speaking to.