I have a bag. 改为否定: I have not a bag. I don't have a bag. 哪个对?

I don't have a bag这个对，
have 的否定是加助动词不是加not

后者对，如果用前者应该是I have no bag。

第二句 I don't have a bag 对。
这是一般时，在这里 have 是一般动词=do，因此否定句用 don't
简例正句为：I do sth ,则否定句为：I don't do sth。
如果将原句改成完成时：I have had a bag.(I have done sth)
则否定形式为：I have not had a bag.(I have not done sth)
因此搞清楚 have 的功能就不容易发生错误。

I have a bag.
改为否定: I have not a bag. I don't have a bag. 俩个都对

都对
若have后直接加not,代表你把它看做情态动词
如果加do否定，则是行为动词

I don't have a bag. have是动词，需用助动词。也可以说I have not got a bag.这与I don't have a bag.是同义句。