一、用was造肯定句
昨天他曾去过图书馆，这一点是肯定的。We can say for certain that he was in the library yesterday.
二、用was造疑问句
他是昨天离开的吗？Was he the one who left yesterday?
三、用was造否定句
他昨天不在家，他去图书馆了。He was not at home yesterday. He was at the library.
四、用was造反义疑问句
昨天他在图书馆，不是吗？He was in the library yesterday, wasn he?
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