回复：what to do? Canadian citizen forgets to carry US green card.

You may ask someone to send the green card to you and then, you can come back, or go to US Consulate to get the Transprotation Letter to come back if you have the evidence that you are US lawful permanent resident. If you just use your Canadian passport to come back, you will be considered to get the B2 visa to come back, and thus, your US green card will be automatically invalid (cancelled).