The only time that happens is when you're holding Command (Apple button).
If you aren't pressing it, try taking the key out and cleaning it.
It's trying to paste something in your typing field, a shortcut to going to Edit>Paste, so it's highlighting that you're actually going to the Edit menu.
when pressing either cmd buttons or the button that does'nt work i get a beep as if it works but no letter, its as if its been assigned to do something else, when pressing the button the edit in the tool bar is then high lighted? when i press the button that doesnt work and tap N a new window appears??
Create another account; login with that account and see. If it works normally, then you did. if it still doesn't work, then you have a hardware issue. Unless you installed a 3rd party app, I doubt you remapped the keyboard.
Do you go see a doctor or explain things over the phone/Internet? Do you go to the mechanic or ask them for help over the phone/Internet? See where this is going?
You could take the machine to an Apple store if you are in the US. They are still open and could answer all of your question.
Lastly, without know what model the Mac is, how could one say how much it is. If it is a desktop model, then it is just a keyboard. If it is a notebook; then it will be more expensive.