There will be rain 为什么将‘There ’改成‘It ’后‘rain ’就要加上‘ing’ 变成It

There will be rain 为什么将‘There ’改成‘It ’后‘rain ’就要加上‘ing’ 变成It It will be rainy.There will be rain.为什么一个是rainy 一个是rain It will be rain 能不能改成it will rain there will be rain There will be rain tomorrow 里的rain为什么不能改成rainy? does it often rain there in spring为什么用rain there will be ___tomorrow.(rain) look,it (rain)there now. There's no （rain） in the south of China today.But it will be It is reported that there will be a heavy rain ___ the hike It was f____ y_____ morning .But today there will be rain an There's no ___ today.But it will be ____(rain) tomorrow.