MacBook Air 2018 typing wrong letter
I would suggest you check your keyboard with OnScreenKeyboard. You can hit one letter C and see whether it is pressing on correctly or not. 

To enable the onscreen keyboard in case you don't see it:

Open System Preferences and go to “Keyboard”, and look under the “Keyboard” tab

Check the box next to “Show Keyboard &amp; emoji viewers in menu bar”

Pull down the newly visible Keyboard menu and choose “Show Keyboard Viewer”

After you conclude with this, you can try the following solution, it is a bit weird but have seen working for few people.

Weird solution

Let me know how it goes.