1.He can speaks English well 2.they must don✀t do that again. 3.you had better to do it at once.

1. speaks改为speak(can后面接动词原形）
2. must don't改为mustn't
3. 去掉to (had better后接动词原形）

都有错：
1 speaks改为speak
因为情态动词后接动词原形

2 must don't -> mustn't
含有情态动词的句子，变否定句时是在情态动词后加NOT。

3 去掉TO
因为had better 后接动词原形。

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1. speaks 改为 speak
情态动词 can 后面跟动词原形。

2.must don't 改为 mustn't

3.去掉 to
had better do sth 最好做某事

都不对
正解：1. He can speak English well。
2. Tthey mustn't do that again.
3. You had better do it at once.

情态动词后面用动词原形，否定直接加not不用助动词
1.He can speak English well
2.they mustn't do that again.
3.you had better do it at once.