isn't ：He isn't a teacher. 他不是老师 说明身份和 Heis outgoing. 他性格外向. 描述性格特征居多
doesn't：He dosen't eat meat as he is a vegeterian.因为他是素食主义者所以他不吃肉 一般用于表达某人的意志意向
didn't是doesn't 的过去式 ,一样的用法.don't 用于第一和第二人称 而doesn't 用于第三人称,例子：I don't like swimming. He dosen't like swimming.
not:I am not John.