My partner is not Canadian and we moved away from Canada in August. Do we have to file as common law or can we file as single? We became common law in April.

If you became common law in April 2020, you would not report that you are common law on your 2019 tax return. Instead you would wait until you do your 2020 tax return next year.

If you became common law in April 2019, then you would report that you are common law on your 2019 tax return, as well as the date that your martial status changed.