How do you tell if the words indicating the sentence is a question are at the beginning or end of a sentence?
Khi nao=when is always in a question... Sometimes it comes at the end, but often at the beginning of a sentence
And I wonder when should put khi nao ahead and when should put it at the end
I found this in a grammar book: Question words for when? (bao giờ? khi nào? lúc nào?) can be placed either at the beginning or at the end of a question. When used at the beginning, these indicate present or future tense. For example: Bao giờ chị đi Hà Nội? When is she going or will she go to Hanoi? Khi nào mẹ về nhà? When is she returning or will mother return home?
When used at the end they indicate past tense, like:
Anh ấy đi Việt Nam bao giờ? When did he go to Vietnam?
I hope this helps.