My phone has 8GB internal storage and I can only use about 3GB. The rest is inaccessable even after factory reset. I once rooted and used a software that make partitions to my storage. After I signed in into secondary partition then the phone is locked so I have to factory reset it. Since then I cannot sign in to the former partition or delet that space. Now I haven't root my phone but it says It is rooted and no matter how much I factory reset, it didn't change.
Once a phone is rooted you can't actually do a true "Hard Reset". You have to unroot it first.
Now, none of this will give you more "Internal" storage. That number you see of 8GB is the full amount the phone has, before the OS and anything else was installed. The only ways to have more space is to add a SD card, which will give you "External" storage, and if the phone is capable, convert the card to "Internal" storage. However, that has its own risks and limitations as well.
As to the rooted phone problem, I know of two apps that should be able to help with unrooting a device. Others may know of more or other ways, as I am really not a fan of rooting, so not my expertise. The two apps, which should be able to be found at the Google Play Store are "SuperSU" and "ES File Explorer". I think "SuperSU" is free but I don't know about "ES File Explorer".
How to unroot Android: Using SuperSU
Download, from the Google Play Store, and install "SuperSU".
Launch the app and go to “Settings”.
Scroll down and locate, and tap, “Full unroot”.
It will ask you to confirm that you really want to unroot your device. To start, tap continue.
When it is done, the app will automatically close and then restart your device.
When it is done, and the device restarts, it should be fully unrooted.
Uninstall "SuperSU" and you are done.
How to unroot Android: Using ES File Explorer
Download, from the Google Play Store, and install "ES File Explorer".
Launch the app and tap "Menu".
Scroll to locate “Tools”. Tap that and then turn on “Root Explorer”.
Now you will have to grant it "root privileges", if prompted.
Go back to your home screen and then navigate to your device’s root folder. It should found as “/” in explorer.
From root, navigate to “system” and then “bin”.
Find, and delete, the “busybox” and “su”. If you can’t find them, move on to step eight.
Navigate back to “/” and open the “app” folder.
Restart your Android device. Your device should reboot unrooted.