天长地久 forever
Explanation
形容时间悠久；也形容永远不变（多指爱情）。
Describes a long time; also describes something that is forever unchanging (mostly in reference to love).
Origin Story
传说中，一对恋人为了表达对彼此永恒的爱，在山顶上刻下“天长地久”四个字。他们的爱情故事流传至今，成为一段佳话。时光荏苒，山顶上的字迹渐渐模糊，但他们的爱情却依然在人们心中流传。后人为了纪念这段爱情，在原地建了一座寺庙，香火不断。 很久以前，在人迹罕至的深山里，住着一对相爱的情侣。他们为了躲避战乱，隐居于此，过着平静而幸福的生活。男的叫天，女的叫地。两人相爱至深，他们的爱情故事传遍了整个村庄。他们每天都一起在山间田野里劳作，一起在星空下赏月，一起在小河边嬉戏。他们的生活虽然简朴，但却充满了甜蜜和快乐。他们约定，要永远在一起，他们的爱情要像天和地一样，永远存在。 一天，他们来到了一块巨大的石头前，天用随身携带的刻刀，在石头上刻下了“天长地久”四个字，以此来见证他们的爱情。他们相信，只要他们彼此相爱，他们的爱情就能像这块石头一样，永远存在。从此以后，天长地久就成为了他们爱情的象征，也成为了后人传颂的佳话。
In legend, a couple carved the four characters "heavenly long and lasting" on a mountaintop to express their eternal love for each other. Their love story has been passed down to this day, becoming a beautiful story. Time flies, the inscription on the mountaintop gradually fades, but their love still circulates among people. In order to commemorate this love, later generations built a temple on the original site, with continuous incense.
Usage
多用于表达爱情的永恒和长久，也可用于其他方面，比喻事物持久永恒。
Mostly used to express the eternity and longevity of love, it can also be used in other aspects, to illustrate the lasting and eternal existence of things.
Examples
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他们的爱情天长地久。
tāmen de àiqíng tiānchángdìjiǔ
Their love will last forever.
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这份友谊天长地久
zhè fèn yǒuyì tiānchángdìjiǔ
This friendship will last forever