If they left now,they would arrive in good time.
从条件从句看,“如果他们现在离开”,用一般过去时,表示与现在事实相反,又因为结果主句的主语是they,那么应该用would+动词原形的形式,用虚拟语气.
与现在事实相反的虚拟语气的规则是：
条件从句形式为：If (we,you,she/he,they)+动词过去式（be用were）
结果主句形式为：I（we）should
You would
She/he would + 动词原形
They would