Had began or had begun

Had began or had begun?

what is the right way to write this sentence -
We had [began/begun] working on it a week ago.

and why?

thanks.

Other Answers:

had begun

Why? Because the irregular verb to begin is conjugated as follows:
begin (present or infinitive) - began (simple past) - begun (past participle)

Begun... ??? Hopefully.. D: I hate confusing grammar

had begun. it's past progressive.

Had begun.