Hi, Anewcomer, If you simply want to make a question, then the right sentences are these ones (bold): " why doesn't our school have a canteen " and "why our school doesn't have a canteen" "What he is trying to do" and " what is he trying to do " "What school is the best" and " what is the best school " The other ones are correct if it's a complex sentence such as: I have no idea why our school doesn't have a canteen . It is difficult to understand what he is trying to do . I have to find out what school is the best .
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