wǒ bù dǒng: means I don't understand( the deep meaning of a situation, theory，knowledge...) can't simply translate to 'I don't know'. We can't say: I don't know how to get there.( wǒ bù zhī dào zěn me qù nà.) I don't know Richard.( wǒ bù rèn shi Richard.) But you can say: I don't understand what you mean.( wǒ bù dǒng nǐ zhè shì shén me yì si.) I don't know Chinese ( in knowledge) (wǒ bù dǒng zhōng wén.) wǒ bù dǒng le: 'le' after a verb generally means that action finished. So this sentence means: I thought I understand it, but now, I'm confused, I'm lost, I don't understand it now. nǐ dǒng le ma? ( Do you understand them now?)

"wo bu dong "is usually used to espress a completed sentence and the fact.But "wo bu dong le " is usually use to show the doubt of someone, followed by a reason.By the way, in Chinese "le" is put at the end of a sentence. It means that you have finished something.

both of them mean i dont know,if somebody asks 你懂了吗(do you understand)？you can answer wo bu dong,but you cant say “wo bu dong le” when you're talking about sth with someone,after he fisnished what he wanted to say(he didnt ask if you understand it), and you dont get it,you can say 我不懂了+为什么（why）。。。。

Wo3 bu4 dong3。-- I don't know. I don't understand. Wo3 bu4 dong3 le。-- ‘le’ here, the same with the one in this sentence: "Xia4 yu3 le。", emphasizes that the referred thing or the situation has already changed. Wo3 bu4 dong3 le。-- I understand sth. at first, but now I am confused, I'm lost, I don't understand, since......

In Chinese, people sometimes like to end a sentence with a sound. Usually these sounds don't have much meaning. I guess their existence is to make up for the lack of undulating intonation in Chinese and to help people distinguish the type of the sentences they hear. For example, if a sentence ends with a question mark, one would usually hear a "Ma" sound. When it ends with an exclamation mark, sometimes one would hear a “Ａ” or "Ba" sound. I agree with 玲. I don't see the difference ne :D (except perhaps the later one sounds cuter )