It's very understandable how new content words emerge in a language, since we can see it happening constantly in the modern day. On the other hand, I have trouble imagining the process by which a word like 被 (which is a particle marking a passive sentence in Chinese), can come to be added to a language.
Is there a good explanation of this process?
The process is known as "grammaticalization", and there is a ginormous literature on the topic. There are very many sources: here are a couple of examples. Many languages have a an expressive utterance like "pah!" or "bah!" signaling rejection – it's not even language. But it has become a post-verbal negative marker in a number of Lacustrine Bantu languages, for example Logoori mba. There is a verb tak "want" in many Bantu languages: it gets shortened to ta and is the future prefix on verbs in Swahili (also Shona). This is related to the use of "will" (want) as a future marker in English.
