All other pages have a left menu. I mean "siderbar" from the menu. How can I edit the tags page to be a left sidebar?
rhymebull you need to cancel out this CSS https://github.com/flarum/tags/blob/v0.1.0-beta.15/less/forum.less#L57-L77
One option is forking the extension and modifying the file.
Another would be to add additional CSS that will take priority over that one. This is the original CSS that was itself overridden by the Tags extension https://github.com/flarum/core/blob/v0.1.0-beta.15/less/common/sideNav.less
You probably only need to copy and override the 8 properties that are customized in the Tags extension back to their value from Flarum's original CSS.