OK lets checkout somethings here:
First go into your systems preferences Keyboard control panel. There check off Show Keyboard & Character Viewers in menu bar Then after saving your change you should see a Icon in the right hand side of the menu strip with an asterisk in it. Click on it to open the keyboard viewer press your keyboards I key does the key on the viewer light? Also try using your mouse to hit the viewers I Key, does the letter correctly show up in your text?
If the I key is correctly working here (just popping up the alternate key menu) then we need to check the Language & Region control panel. Make sure you have the correct language listed (remove any others). Then go into the Keyboard sub menu here and go to Input Sources (you can also get to this from the keyboard control panel I do it this way as it forces me to check the language setting) do you have the correct keyboard showing in the listing if you have others I would remove them. If you make any changes you will need to restart your system.
Also check Shortcuts sub menu as well to see if someone remapped something.
Is it fixed? Or if you noted the keyboard button didn't work correctly then you need a new keyboard. Let us know what you encounter if you still are having problems.