什么时候用isn't 或didn't

我是英语老师，我来帮你解答吧。英语中如句中原来就有be ，也就是（am,is,are，was,were)时，改为否定句直接用isn't, am not, aren't, wasn't 和weren't 。如：He is a teacher.否定句：He isn't a teacher.当句中没有be 动词时，改为否定句就要加助动词do, does,did 来帮忙。如：He comes from England,否定句：He doesn't come from England.再如：He came from Beijing.否定句：He didn't come from Beijing.明白了吗？希望我的耐心回答能令你满意哦！