This 汉代 poem is about a woman who promises her everlasting love to her 君子. The style is called 乐府, meaning that it’s composed in the style of a folk song. The most famous poems written in this style are 《陌上桑》 and 《孔雀东南飞》, but 《上邪》 is a little bit shorter (=easier to memorize).
上邪
上邪！
我欲与君相知，
长命无绝衰。
山无陵，
江水为竭，
冬雷震震，
夏雨雪，
天地合，
乃敢与君绝！
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In modern Chinese, from 百度百科：
天啊！我愿与你相爱，让我们的爱情永不衰绝。除非高山变成平地，除非江河干得不见一滴水，除非冬天打雷，夏天下雪，天和地重合到一起——到那时才敢对你说出一个“绝”字。