How should I understand the next sentence "Nature forswears antiquity" properly？

Well like most poetry, it's up for interpretation. My own view is that it's a reflection on the transience of nature. Nature isn't around for hundreds of years, it is around for a short time and then it goes away. Flowers more so than other nature. In that way, "they foreswear antiquity"

I love the Du Fu poem! I like the first translation yes, but both have their merit.

Great share!