Some people get confused about using passive .
In a passive clause we usually use “by” to introduce the agent .
Your mother caused a lot of problems . => a lot of problems were caused by your mother .
We used were +P.P (past participle) in order to make passive form of sentence.
Be+ P.P it’s a good formula in order to make a passive form
Am,is,are,was,were + P.P is a general form of previous formula
John opened an account in national bank => an account was opened by john .
Those pyramids were built around 400 AD .
This kind of passive is the easiest one.
You can refer to an informative and nice book : Practical English Usage (Third Edition )