Having your mail sorted into categories isn't for everyone and can be confusing if you haven't fully understood how this feature works. If you want to turn it off, don't look for this in the Settings app. The switch you need is in the Mail app itself. In the Mail app, go to your Inbox list. There will be a more button, three dots in a circle, near top right of the screen. VoiceOver users can probably locate this with a few swipes right from the top of the Inbox screen or by exploring the top right corner of the screen. Use the more button and select List View from the menu that pops up. VoiceOver users please note that List View isn't described as a button, check box or whatever; you will just hear its name spoken and should then double tap to select it. Job done!