He was at the police station yesterday.（改为一般疑问句）

Was he at the police station yesterday?1、谓语动词是be动词、助动词、情态动词时,只要直接把这些词置于句首,句末改成问号。2、谓语动词是行为动词时,必须在句首加上助动词Do、Does（三单）、Did（过去式）加上这些助动词后,句子中谓语动词必须用原形,句末改成问号。