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Once you are common-law and separated for 90 days you will then be considered separated and not "single".
If you file your return before the 90 day- separation period (which includes Dec 31) is over, you should continue to indicate a marital status of living common law. As you said, your status changed in August. You must tell the Canada Revenue Agency (CRA) about your new marital status by the end of the following month after your status changed.
If after filling the return you continue to live separate and apart from your common-law partner for at least 90 days, you should complete and submit Form RC 65, Marital Status Change.
For more information, please visit:
Marital Status
Change in Marital Status: Advising the Canada Revenue Agency
