Don't make oaths lightly to the moon, for it is always waxing and waning.

Don't make oaths lightly to the moon, for it is always waxing and waning.

不要轻易对月亮起誓，因为它总有阴晴圆缺。

解析：oath(n.)誓言；wax(v.)变大、变满，指月盈；wane衰退、亏缺指月亏。