It is definitely wrong to say 你明白我说吗？ 什么 is necessary when it comes to the question begins with what. And 吗 is just a modal particle, usually it is placed in the end of the sentence.

1,、u can say "我说的你明白吗"？ 2,、我没有去过中国 3,、i'm not sure what's the meaning of "他她们说" but "他对她们说(he has told to the girls)" or “他们/她们说” regards to u .

1) I asked a chinese young man how to say "do you understnad what I say?" in chinese and he told me that's "你明白我说什么吗？". My question is: is that correct? Can one use both 什么 and 吗 together in the same question?? I thought one could only use one at a time? Would it be wrong if I said "你明白我说吗？" ?? Brian's comment: the Chinese young man translated correctly. '吗' usually is added at the end of question no matter '什么' is in the same sentence or not. 2) I saw somewhere the following sentece "我没有去中国" and it was translated as "I haven't been to China". My question is: is it correct? Or is it just an incorrect literal translation?? Brian's comment: it is not exactly from my personal point of view. I haven't been to China=我没有去过中国, and 我没有去中国=I didn't go to China. 3) I saw the following sentece "他她们说" and as I couldn't understand what it meant, I decided to use the google translator. It trasnlated the sentece as "he told the girls...". My question is: is that correct?? Brian's comment: 他她们说>>.it looks like a typo I think. it should be '他们说,or 她们说 or 他/她们说'