so i have a command it has if (!$this->testPermission($sender) return false will this stop right there and not execute further code compared to if i return true?
Assuming you mean to use the hasPermission() method, it depends whether or not they have the permission to return true or not. If they do not have the permission, it will return false. We do not have any else statement or an else if statement to say what the other output would be, so it would probably just be null, if I’m not wrong.
The value you return in the scope of onCommand will determine if command's usage will be sent or not. You should learn what 'return' does