英语改否定句的口诀如下：

在助动词前加 not

例如：I am going to the store. → I am not going to the store.

对于一般现在时和一般过去时，在主语后面加上 not

例如：She sings beautifully. → She does not sing beautifully.

对于情态动词，在动词前加上 not

例如：You can speak English well. → You cannot speak English well.

对于某些动词，需要在前面加上 do 或 does not

例如：He studies hard. → He does not study hard.

对于be动词，在动词后加not

例如：is not, are not, am not

对于情态动词如can, should, will等，在动词后加not

例如：cannot, should not, will not

对于一般现在时和一般过去时的动词，在动词前加 don't 或 doesn't

例如：She sings beautifully. → She doesn't sing beautifully.

对于第三人称单数（he, she, it），在动词前加 doesn't

例如：He plays basketball. → He doesn't play basketball.

对于某些固定表达，如 always, ever, never 等，变否定句时改为相应的否定词

例如：He always goes to bed early. → He never goes to bed early.

对于祈使句，在句首的动词前加 Don't

例如：Let's go to the park. → Don't go to the park.

这些口诀可以帮助你快速将英语句子改为否定句。希望这些对你有所帮助!