Does turning on Wi-Fi and using it on your mobile use my prepaid data?

Hi, I'm just curious does turning on Wi-Fi use my prepaid data. I turned on Wi-Fi on my mums phone and my mums on prepaid. I started watching youtube videos and in 1 day a text is sent to my mum that she has to recharge but she recharged 5 days ago but she usually recharges once or twice a month. My mum has the nokia lumia 520. I'm thinking of getting a 720 but i have plans to get an iphone 5c with a 4g or 3G plan. So does using internet affect my prepaid data?

Wi-Fi is good only when connected to the local wireless internet connection. Just because it is on, does not mean it is always connected to an wireless internet connection. Wireless Home router connected to the internet is a must have or any other hotspots. If properly connected via Wi-Fi to those, then the data being used should not be charged to your plan, prepaid or not.

No, but always double check your on Wi-Fi cuz you may lose Wi-Fi connection and spend your own data

Nope, if you are connected to the Wi-Fi and you don't turn on your data, then you are using Wi-Fi.

Well it will not. But if you keep your phone data enable, when your Wi-Fi signals are weaker, phone automatically use the prepaid credit to connect to the internet.
Best thing is to disable your phone data option ( leaving Wi-Fi option on) when you brows the internet.
Other than that most modern smart phones have auto updates on the applications & software on the phone. If you leave the phone data enable then the phone use the prepaid credit to update its applications & software's hence you may loose your credit eventually…
you can disable or enable mobile/phone data under phones settings >mobile network>data connection.

No. Wi-Fi does not use data.