Complete the sentence with the definite consequence using would, not a possibility with could or might: We missed the midnight ferry. If the taxi had taken the tunnel, we ___ before the gate closed. (board)

Explication

Would have boarded expresses the definite unreal past consequence the sentence asks for; could have boarded or might have boarded would state only a possibility, which the instruction excludes. Would board would leave the consequence in the present or future despite the completed midnight journey.