Many students( )(talk)when the teacher( )(come)into the clas

Many students( )(talk)when the teacher( )(come)into the classroom yesterday.
原因理由,
括号里填什么,为什么这么填

Many students was talking when the teacher came into the classsroom yesterday
许多学生正在说话,当老师走进教室的时候
老师走进教室是过去发生的,故用过去式came
而学生说话,是老师走进教室那时正在进行的,故用过去进行时was talking