前天是晴天但是后天要下雨用英文怎么说

"前天是晴天但是后天要下雨" 用英语可以说成 "It was sunny yesterday, but it's going to rain tomorrow."

"前天" 的英文是 "yesterday"，表示 "昨天"。
"后天" 的英文是 "tomorrow"，表示 "明天"。
"晴天" 的英文是 "sunny"，表示 "晴朗的"。
"下雨" 的英文是 "rain"，表示 "下雨"。
"要" 的英文是 "going to"，表示 "将要"。

例句:

"It was sunny yesterday, but it's going to rain tomorrow." (昨天是晴天，但明天要下雨。)
"It rained last night, but it's going to be sunny tomorrow." (昨晚下雨了，但明天要晴天。)
"It's supposed to rain tomorrow, but it's sunny today." (明天应该要下雨，但今天是晴天。)