If you've received (been "served" with) a Statement of Claim in Ontario, and your name is listed as a Defendant, is means you have been sued in the Superior Court of Justice.
You have 20 calendar days from the date that you were served to respond to the Statement of Claim. That being said, the deadlines can be extended with the help of a lawyer. Lawyers are required to comply with the Rules of Professional Conduct, which require lawyers to agree to reasonable adjournments and waivers of procedural formalities in many circumstances. Therefore, if both sides are being represented by lawyers, the ordinary deadlines can be extended. If you have retained a lawyer, your lawyer can also provide the other side with a Court document called a Notice of Intent to Defend, which must also be filed in Court. This gives you another 10 days in addition to the original 20 days to submit a Statement of Defence.
Your response is called a Statement of Defence, and it must be filed in Court, as well as delivered to the Plaintiff or the Plaintiff's lawyer. Your lawyer will also need time to evaluate your case, and prepare a defence.
The Statement of Defence will indicate which parts of the Statement of Claim you agree with, which parts you disagree with, and which parts you have no knowledge of. It will also include your own version of the facts that form the subject matter of the lawsuit, and you can dispute any statements that you disagree with.
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