该句子改为否定句是：I didn't do well in the race.（我在比赛中表现不佳。）
英语中的句型转换属于较难的题型，对孩子的英语综合能力有一定要求。除了必须掌握基础的词汇外，还要对语法知识和句子语序有很好的掌握，很多孩子在这种题型上失分比较严重，其实是他们没有掌握好方法。
肯定句变否定句变法规则 :
一 、有 be 动词( am 、is 、are )和情态动词( can 、could 、may 、must 、shoud 、 will 、 would 等)时，直接在be动词和情态动词后面加 not ，some变为any，and 变为 or ，too 变为either ，其他的照抄。
例如 :
①、I am a student. – I am not a student.
②、Amy is a good girl.–Amy is nota good girl.
③、These flowers are very beautiful.–These flowersare not very beautiful.
④、I can swim.–Ican't( can not ) swim.
⑤、There are some books on the table.–There are not anybooks on the table.
⑥、Jack is a doctor , too.–Jack is not a doctor , either.
⑦、I was a student ten years ago.–I was nota student ten years ago.
注 :⑴、 some 、and 用于肯定句中，any、or用于否定句和疑问句中。
⑵、was 是 is 和 are 的过去式，were 是 are 的过去式。
二、没有 be 动词( am 、is 、are )和情态动词( can 、could 、may 、must 、shoud 、 will 、 would 等)时，在动词前面加 don't 、doesn't 、didn't ，动词恢复成原型，some变为any，and 变为 or ，too 变为either ，其他的照抄。
例如 :
①、I like math.–I don't likemath.
②、Jack likes math, too.–Jack doesn't like math , either.
③、They like bananas and apples.–They don't like bananas orapples.
④、Lily gets up at six every morning.–Lily doesn't getup at six every morning.
⑤、His father likes playing football.–His father doesn't likeplaying football.
⑥、I went fishing yesterday.–l didn't gofishing yesterday.
注 : ⑴、当句子是一般过去时态时，在动词前加 didn't ，动词恢复成原型。
⑵、当句子是一般现在时态时，主语是第三人称单数时，在动词前加 doesn't ，动词恢复成原型;主语不是第三人称单数时，在动词前加 don't。
