【小题1】We heard (听到) him singing in the next room when we passed.
【小题2】The tired children don’t get home until （直到）7 pm.
【小题3】While the boy was heard (争吵) with his sister, their mother came into the room.
【小题4】The dog has three legs. How arguing (奇怪的) it is!
【小题5】Many athletes give money to schools and charities, and do a lot of work to help people.
This is a great strange (机会) that many people do not have.
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解析：
【小题1】本句的从句为一般过去时，故主语也应该用过去式，故本题空格处填听见的过去式heard。
【小题2】本题的含义为疲惫的孩子们直到晚上7点钟才到家，not……until表示直到……才的含义，故本句空格处填until。
【小题3】while后引导的从句为过去进行时，结构为was+现在分词，故本句空格处填争吵的现在分词arguing。
【小题4】be动词后跟形容词，在感叹句中，how后跟形容词，故本句空格处填奇怪的形容词strange。
【小题5】本句的含义为这是许多人没有的极好的机会，冠词a后跟名词，故本题空格处填机会的名词形式chance。