I get "you do not have permission to write to this folder" (or something similar)
As a multiuser operating system with some sense of civilized conduct, Linux (Unix) is designed to thwart your unauthorized attempts to tamper with resources that may belong to others, or that are vital to the proper operation of the system.
Only the 'root' user of the system, or a SuperUser, may freely meddle with everything. If you're the sole user of your PC, then you're the boss, but Bianca wants some assurance that you know what you're doing. She gets it rough, sometimes, so be gentle.
Whenever you want full authority to corrupt your file system, prefix your Terminal commands with the word sudo
. For example, to invoke the file manager with SuperUser "permissions", you type sudo nautilus
. Then you can copy files anywhere.
You'll be asked for your password each time you use sudo.
Be careful about modifying existing files.