正确形式If it {rain}__last night ,the ground would be wet and the temperature would be low today.

应填 had rained.因为这是一个考查虚拟语气的题,而且是一个混合虚拟句（主句和从句的时间不同）,从句中表示的与过去情形不符,从句中应用“过去完成时”,见 last night.主句中表示与现在的情形不同,所能应用“would /...那就是对过去事实相反的假设，那么主语应该用would have done的啊，那此句怎么不用would have been wet 呢，比较困惑！因为主句中指的现在的情况呀，所以要用 would / should / could / might + 动词原形，你所说的那是主句中也表示与过去的情况的相反，才用 would have been 的形式。不明白，望继续追问。