Do I have to answer questions sent to me by a defense attorney if I’m the plaintiff?

Yes, you need to answer those questions.  Those questions are interrogatories which you need to answer and sign under oath (penalty of perjury).  Those questions are part of the discovery process in which each side tries to obtain information about the opposing party's case.  Those are probably form interrogatories.  You can also be sent special interrogatories which are additional questions pertaining to your case.  Other forms of discovery which may be sent to you could include request for production of documents, request for admissions and possibly a subpoena.  After the initial phase of discovery, the opposing party will probably take your deposition where you would be answering questions at the opposing party's office under oath with a court reporter present. The purpose of the deposition is to obtain additional information beyond the responses you have provided in answering discovery.  You can send the defendant interrogatories, request for production of documents, request for admissions, etc.  You won't be able to issue a subpoena since you are not an attorney.  If you want to issue a subpoena, you would need the judge in your case to issue it.  If you don't answer the interrogatories or other discovery, the opposing party can file a motion to compel further responses and request sanctions which are court imposed fines.  When answering the interrogatories or other discovery, you can object to answering a question and state the specific legal ground(s) for your objection to a particular question or particular questions.