Changing taps in bathroom sink but can't turn the shut off valves and have tried wd

https://www.youtube.com/watch?v=yDRXyGUtrf4

See if this helps.

The video is pretty good but can someone explain to me how a washer, inside a valve that is constantly in water, dry out. More likely the packing material around the valve stem has adhered to the stem itself.
Even more likely that there was a leak round the valve stem and someone tighten the bonnet nut excessively.

I suggest loosening the bonnet nut (that nut behind the handle and around the stem) one half to one turn before putting pliers on the handle. Less likely to break something that way.

If the shutoff valves have not been used for long time mineral will build around the stem and will make the washer hard and not able to turn the valve, please open the link below and also see the image below to show the internal parts of the valve, Thanks.

John

Stuck Shut-offs | Plumbing | Plumbing, HVAC & Electrical | This Old House