“The citation shall include the following notice to the defendant: ‘You have been sued. You may employ an attorney. If you or your attorney do not file a written answer with the clerk who issued this citation by 10:00 a.m. on the Monday next following the expiration of twenty days after you were served this citation and petition, a default judgment may be taken against you.’”
Texas Rule of Civil Procedure 99.c.
“Where citation has been personally served at least ten days before appearance day, exclusive of the day of service and of return, the answer of the defendant shall be filed at or before ten o'clock a.m. on such day. . . .”
Texas Rule of Civil Procedure 535.
The answer that must be filed with the court is nothing more than a properly-formatted legal document that tells the court that you do not agree with some or all of the statements contained in the petition that was filed by the plaintiff and with which you were served.
Filing a legally-sufficient answer — in a timely manner — is necessary to protect your right to tell your side of the story in court.
A word of caution: Although timely filing an answer will usually protect your legal rights, there are circumstances where filing an answer may actually WAIVE certain rights or defenses that you may have to a lawsuit.
The best course of action, of course, is to always obtain competent legal advice and representation as soon as possible, which often may be prior to the filing or service of a lawsuit.
But at a minimum, you should contact an attorney as soon as possible after being served with a lawsuit. Any delay beyond this point in time may make it difficult — or possibly even impossible — to find an attorney who will have the time to help you.