Wǒ bù dǒnɡ.
我不懂。
I don't understand it.
Notes:
我(wǒ): I
不(bù): not
懂(dǒnɡ): understand
Example:
A:你懂吗?
Nǐ dǒnɡ mɑ?
Do you understand?
B:我不懂。
Wǒ bù dǒnɡ.
I don't understand.