天长地久的英文：
[Literal Meaning]
the sky/long-lasting/the earth/long-lasting
as long-lasting as the sky and the earth

[解释]
久：长久。跟天和地一样长久。形容时间很长。

[Explanation]
to be enduring as the universe

[例子]
每个人都向往着天长地久的爱情，可是很少有人愿意改变自己适应对方。

[Example]
Everybody longs for an everlasting love, but few people are willing to change to adapt to the other.

[英文等价词]
as eternal and unchanging as the universe

Our friendship is everlasting and unchanging.
我们的友谊天长地久。

I love you forever and always will
我爱你天长地久

May our friendship last forever.
愿我们的友谊天长地久

Sincere friendship is everlasting and unchanging.
真挚的友谊天长地久。

A man should keep his friendship in constant repair.--Samuel Johnaon, British writer
只有经常“浇灌”,方能保持友谊天长地久。英国作家约翰逊