Welcome to the forum
There's a big difference between apps and other files, so I'll consider these separately.
The main purpose of the sd card in Android is media storage. So if you are having trouble with that then it would help to know what you are trying to move, how you are trying to move it, and what OS version you have (what device may help too). The main tool for moving files/folders is a file manager (also known as a file explorer). Your phone may have one, otherwise there are plenty in the Play Store. I personally use X-plore, Solid Explorer or Root Explorer. These are similar in concept to Windows Explorer on a PC or Finder on a Mac. I would recommend copying stuff rather than moving if you have problems (you can always delete the original if the copy succeeds, it's just safer).
OS version is relevant because in Android 4.4 Google went too far with file security, making it hard for most apps to write to the sd card (though a built-in file explorer should still be able to). In 5.0 they provided a way for third party apps to write to anywhere on the card again, but the app has to ask for that permission, so if the developer didn't implement that it still won't be able to.
Now the above is fine for music, ebooks, etc. Any remaining problems after moving files may be because the app you want to view them with needs to be told where to find them. In which case there should be an option in the app's settings. But apps are different.
You can't move an app the same way because (a) you can't even access where it is installed unless you are rooted, and (b) if you were to move the app (or bits of it) that way the phone wouldn't know where to look for them and you'd just break the app. Again your options depend on OS version, but also on what the manufacturer provided.
First thing, go to Settings, Apps, Downloaded, select an app. Is there a "move to sd" button? If there is then that's how you can do it. It may be greyed out, as not all apps can be moved (it's up to the developer) but you have the ability. Pre-installed apps cannot be moved, but they live in a different partition anyway. Not all phones have this option though, as it was deprecated in 4.0 (but many manufacturers retained it). It also only moves part of an app.
If you have 6.0 or later you may have the option to format the card as internal storage. If so it will then behave as internal storage, so anything can be stored there. It will also be reformatted (so back up first) and encrypted (so you won't be able to use it in another device). This is only recommended if you have a fast card (any sd is slower than internal storage, a slow card will slow the phone if used this way).
Anyway, without knowing what exactly you are doing I think that's all I can suggest. If you still have a problem please give us a little more information about what exactly isn't working.
