明天一定会下雨用英语可以说成 "It will definitely rain tomorrow."
这句话的来源是将 "明天一定" 翻译成 "tomorrow"，将 "会" 翻译成 "will"，将 "下雨" 翻译成 "rain"。
一些英文例句及其中文翻译如下：
"It will definitely rain tomorrow." = "明天一定会下雨。"
"I'm sure it will rain tomorrow." = "我确定明天会下雨。"
"Tomorrow's forecast calls for rain." = "明天的天气预报说会下雨。"
"I wouldn't be surprised if it rains tomorrow." = "如果明天下雨我并不会感到惊讶。"