1. I do not understand.
我不明白(or:我不懂)
wǒ bǜ míng bài(or:wǒ bǜ dǒng)
2.can you please repeat what you said.
请问你能重复一遍吗?(or:请问你能再说一遍吗?)
qǐng wèn nǐ néng chóng fǜ yī biàn ma?(or:qǐng wèn nǐ néng zài shuō yī biàn ma?
3.can you speak slowly.
你能说慢点吗?
nǐ néng shuō màn diǎn ma?