Selenium Test script for authentication

Please help out solve below scenario-- If User id -pk877 Pas -rahu@876 and Submit button

1) Both User & Password are incorrect. Error expected is “Incorrect client Id or Password”.

2) User id is correct but Password is incorrect. Error expected is “Incorrect client Id or Password”.

3)All are correct, user should able to login

Note - in the above invalid cases a alert Will come which we need to dismiss. I need output as error given in error expected.

Kindly right it as simple you can.