Sometimes, we tinker around with various WordPress themes, making adjustments here and there. And before we know, we have modified the original .PHP files so much, that we’re unable to get back to how they were before.
An ideal approach is to create a copy of that theme, and modify the copy instead of the original. The thing is, if you simply copy the files, when you open WordPress and choose the theme to edit you’ll feel confused.
You will see two identical themes, and you won’t know if you’re editing the original or the second copy.
And that’s where you need to read How to make a “child theme” for WordPress. A pictorial introduction for beginners (I like the simplicity of that blog’s permalinks… no obsession over SEO). 🙂