In my shower I removed the hot water valve to replace the washer and decided to replace the seat also, but the new seat will not screw in. How do I fix this?

Is the new seat exactly like the one that you took out. It must me the same diameter and also the same number of threads per inch and the same courseness.

Make sure you have the right seat, Let me know what happens.

The seat is the same size I think that I have damaged the threads. I do not know how to attach the seat to make the water shut off. A plumber said he would have to cut open my tub surround, I am hoping there is an easier fix I have another shower that I can use but the water keeps dripping

Do you have the old seat? If so, try reinstalling it to straighten out any damaged threads. Some places have metal boards with different size holes with different threads to match up seats. You may have the right size but wrong thread. Sometimes the number of threads can be so close you can't tell them apart visually.

Good points all...

I would tell DIYJAN to take the old seat over to a plumbing supply house and see if they can provide a seat with the correct threads. With any luck, that is all required here... ;)

MARK