Formal:
* 我明白了 (wǒ míngbai le): This is the most common and formal way to say "I understand". It literally means "I understand clearly".
* 我懂了 (wǒ dǒng le): Similar to "wǒ míngbai le", but slightly less formal. It means "I understand".
Informal:
* 知道了 (zhīdào le): This is a very informal way to say "I understand". It literally means "I know now".
* 明白 (míngbai): This is another informal way to say "I understand". It can be used in casual conversations.
More nuanced options:
* 我理解 (wǒ lǐjiě): This means "I understand" in a more empathetic way. It implies you understand the situation and the person's feelings.
* 我知道你的意思 (wǒ zhīdào nǐ de yìsi): This means "I understand what you mean". It's useful when you want to confirm you understand the speaker's intention.
The best way to choose the right phrase is to consider the context and the level of formality.