when you turn off cellular data and use wifi, do you still pay monthly?

Your cell phone plan usually has some fixed monthly amount (assuming no enrage charges) which you are always responsible for. Even if you turned the phone off and left it in a drawer all month, you are still obligated to pay your monthly service bill.

Turning off cellular data on the phone has absolutely nothing to do with your agreement with your cellular service provider.

I'm not aware of any supported iPhone carrier on the planet that permits the use of an iPhone without a data plan.

If you use wifi, you are not using your cellular data, but your monthly plan would still be charged even if you don't use it. That's why its a monthly plan rather than a prepaid contract, where you pay upfront, and when your balance runs out you stop getting service.

In that case Using Wifi would keep you from using your prepaid balance.

But again a Monthly contract is charged wether you use it or not for the specified amount of time. Usually for a Year sometimes more.

Yes, you still pay for your phone service, don't you?