question..

[fobboy]

can you please tell me which one is right?

Didn't you add me to the list before?

or

didn't you added me to the list before?

Thanks

[Mariner]

Didn't you add me to the list before?

This stands both for questions, as above, and for emphasis (e.g. "I did tell you to close the door")

[MikeNewYork]

can you please tell me which one is right?

Didn't you add me to the list before?

or

didn't you added me to the list before?

Thanks

After do/does/did one can only use the base form of the verb.