If my charger for my iPhone 7 Plus is not working anymore can I bring in my old charger and get a new one for free?

If it belongs to a recently purchased phone that is under warranty, take it into an Apple store for evaluation. They will then tell you your options.

Yes, if the phone is still under warranty.

It depends, any misuse?

Anyway it cannot be decided by us in this forum

Make a genius bar appointment with Apple and they can tell you more.