几点 position in sentence As I know words describing time are placed before and after Subject, e.g.:


  昨天晚上八点我去超市了。
  
  我昨天晚上八点去超市了。


So I expected the same to be in questions, 


  昨天晚上几点你去超市了？ OR
  
  你昨天晚上几点去超市了？


but I saw something like breaking time before &amp; after Subject:


  昨天晚上你几点去超市了？


Is it grammaticaly correct?
Yes, it is grammatically correct.

And it is very natural.

Breaking it apart (somehow) gives the "eight o'clock" part an emphasis. You can also say 昨天晚上我八点去超市了, if you want to emphasize on "八点".

Question words are sometimes automatically emphasized (as it is what the speaker cares about), so it is very natural to say: 昨天晚上你几点去超市了？