How can I shut off these water valves that won’t turn?

1. Leverage. You could use a pliers or vice grips to give you a longer lever arm. But you have been working out a lot during Covid and you might be strong. Be careful not to crack the handle.

2. Turn off water at the house, unhook hoses and take valves off. Maybe God is showing you in advance that these things are broken, and if your water hoses burst in the dead of night you will not be able to turn them off. Stuck open valve is a good problem to discover in cool blood while the sun is up. You can take them off and mess with them until the valve works again, and then remember to work them periodically so they dont get stuck. Which I am going to go do right now; thank you. Or an opportunity to buy new ones that work as they should.

I would take the plastic handles off, then on many makes of valve the casting makes it obvious which way they should turn.

Then I would get a quality adjustable set so it fits tight in the shaft square and turn appropriately.

Always worked for the ones we had on our systems and we fitted lots of extra valves to be able to isolate sections as needed - they are cheap compared to the time draining and refilling...

Before doing anything - turn off the water further up the line - at the mains if needed.

Those levers are probably plastic, so to eliminate the problem when they break, remove the screws, and there will be a spigot of some shape that they were attached to. A well fitting wrench, Mole or suchlike, or a socket from a socket set, maybe, will give better leverage. Rotate a few degrees at a time, if possible. If still not, then warm up the brass with a heatgun or hairdrier and try again. WD40 probably didn't penetrate but may well do when the levers are off. Bear in mind they will only go 90 degrees - in ONE direction.

As with all water shut-off valves, it's worth closing/opening them at least once a year. Nothing worse than having a leak and finding the main inlet valve (if you actually know where it is..!) is solid - and wide open.