【小题1】I was writing a letter when my mother got home. (对划线部分提问)
What were return to you doing when your mother got home?
【小题2】Could you give the book back to the school library tomorrow? (改为同义句)
Could you if/whether was the book ask for the school library tomorrow?
【小题3】“Are you mad at your sister?” Mary said to me. （改为间接引语）
Mary asked me I mad at my sister.
【小题4】当我们遇到麻烦时，我们应该向警察寻求帮助。（完成译句）
We should What were the police return to help when we are in trouble.
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解析：
【小题1】对做某事提问用疑问词what，后跟一般疑问句，将be动词were提前，故本题空格处填What were。
【小题2】本题的含义为你明天能够把书归还给学校吗？return sth to+地点可以表示把某物归还给某人或某地，故本题空格处填return to。
【小题3】一般疑问句构成宾语从句可用if或whether来引导，表示是否的含义，主句是过去时，从句也要用过去时，故从句的be动词用was，故本题空格处填if/whether was。
【小题4】表示想某人寻求某物可用ask for来表示，情态动词should后跟动词原形，故本题空格处填ask for。