비가 오다.

The Korean word '비가 오다' (biga oda) literally translates to 'rain comes' or 'rain arrives.' It is a compound expression where '비' (bi) means 'rain' and '오다' (oda) means 'to come' or 'to arrive.' The word '비' (bi) for rain has a long history in the Korean language. Its etymology can be traced back to Middle Korean. In Middle Korean, it was also pronounced '비' (bi), and its form has remained relatively stable over centuries. It shares cognates with words for rain in other Altaic languages, suggesting a common linguistic ancestor. The verb '오다' (oda) also has deep roots in the Korean language, signifying movement towards the speaker or a designated point. Its form and meaning have been consistent from Middle Korean to modern Korean. It is a fundamental verb of motion. Therefore, the construction '비가 오다' is a very straightforward and descriptive way to express the occurrence of rain. The particle '가' (-ga) is a subject marker, indicating that '비' (rain) is the subject performing the action of '오다' (coming). Over time, while the individual components '비' and '오다' have maintained their core meanings and forms, their combination '비가 오다' has become the standard and most common idiom to describe rainfall in Korean. There haven't been significant semantic shifts or complex metaphorical developments in this phrase; it has consistently and directly conveyed the idea of rain falling. Its simplicity and directness reflect a fundamental observation of nature. It's a phrase that would have been understood similarly across many generations of Korean speakers, demonstrating the stability of core vocabulary and grammatical structures in the language.