What forms do I need to file in response to an unlawful detainer summons?

When you are served with the summons and complaint (the complaint is the lawsuit attached to the summons), you will need to file an answer to the complaint with the court and serve by mail a copy of your answer on the opposing party's attorney or if no attorney on the opposing party.  You will need to file your answer to the complaint with the court and serve it on the opposing party or opposing party's attorney within the time set forth in the summons.  If you don't file a timely response, a default judgment will be entered against you.  A default judgment means you will have lost; however if that happens, you can file a motion to set aside the default and if granted by the court, the case will then be back on track.

As for the answer to the complaint, at the law library look for answer to complaint in the index of Pleading and Practice.  This will give you the general format for an answer to the complaint.  The answer to the complaint denies the allegations in the complaint.  At the end of the answer is the verification which you sign and date under penalty of perjury.  The verification attests to the veracity of your statements in the answer.  File the answer to the complaint with verification and an attached proof of service with the court and mail a copy to the opposing party or the opposing party's attorney.  You can either use  a court form proof of service or you can write your own.  The proof of service verifies the date of mailing of your documents.  If you write your own proof of service, it just says that you are over eighteen and the attached documents were sent via first class mail unless stated otherwise, to ___________ (name and address of opposing party or opposing attorney) on ________ (date).  You sign and date at the bottom.  The date you sign should be the same as the date of mailing and the same date you file your documents with the court.

Another alternative to filing the answer to the complaint would be to file a demurrer to the complaint.  The demurrer would claim that the complaint fails to state a cause of action (claim) because you are not on the lease and therefore should not be named as a defendant in the lawsuit.  Look for demurrer in the index of Pleading and Practice at the law library to see the general format.  If you file a demurrer to the complaint with the court, you would attach a proof of service and serve a copy on the opposing party or opposing party's attorney as discussed above.