Are you sure there aren't instructions on the summons/complaint as to how to reply - there usually are.
These are JUST EXAMPLES of some replies/answers (your reply may very well be different) -- the SOL has expired (though seems it hasn't in your case), I do not owe the debt, the debt is not mine .......(tell them why you haven't paid)
Be sure to reply within the time limit allowed or they will get a default judgment against you.

