1: Delete It !
If a chunk of code comment or class is not used, don’t comment it out, just delete it.
2: Write clear code.
“make sure you document code that’s hard to understand”. Question: why is the code hard to understand?
Comments can indeed be useful but mostly as a summary of action
4: Don’t Repeat Yourself
Duplication is bad. If you have more than 4-5 lines of code that do the same thing in a single class, refactor to remove duplication.