陈述句变否定句时，原句中：
一、如含有be动词(am, is, are, was, were)、情态动词(can, shall, will, may, should, would, might, had better等)和助动词（will, would, has, have, had）时，要在这些动词后直接加not。如：He is a student.变否定句为 He is not a student.
You can speak English. 变否定句为 You can not speak English.
He has studied English for ten years. 变否定句为：He hasn't studied English for ten years.
二、句中的谓语动词为实义动词。要在句中主句后根据情况分别增加 do not（主语不是第三人称单数，句子为一般现在时）、does not（主语是第三人称单数，句子为一般现在时）或did not（句子为一般过去时），同时，谓语动词要用原形形式。
如： He speaks English. 变为否定句为 He does not speak English.
You eat supper. 变为否定句为 You don't eat supper.
She cried yesterday. 变否定句为 She did not cry yeasterday.
三、肯定句中如果有some，在否定句中一般相应地变为any
对照上述说明，你提问的句子：
We have some lessons at the weekend.
句子为一般现在时，谓语动词只有实义动词have，主语不是第三人称单数。变为否定句，要在主语后加do not：
We do not have any lessons at the weekend.
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