问题的核心在be的变化上.一般现在时的be就用 am,is,are,一般过去时就用was,were过去分词是不会变的.eghe buys three books today (three books are bought today)he bought three books yesterday(three books were ...公式？
宾语变主语，动词变be done，主语前加by 放在句子末尾（可以省略）
上面两个例句的结尾都可以加上by him
其他的没听说过还有什么公式啊......一般现在时 do= be（am isare）+V过去分词
一般过去时 did= be(was were)+V过去分词