Something that happens unbeknown to a person happens while that person has no idea it is happening. You almost never see this word alone. It lives inside the phrase 'unbeknown to me / to him / to her / to them', and that phrase sits inside a bigger sentence about something that was going on at the same time: 'Unbeknown to her parents, she had already sold the house.' So the word is doing two jobs at once. It tells you a fact (she sold the house) and it tells you who was in the dark about it (her parents). Two things matter about how it is used. First, there is a twin form, 'unbeknownst', with -st on the end. Both are correct, both mean exactly the same thing, and 'unbeknownst' is now the more common of the two, especially with younger speakers and in American English. If you learn only one, learn 'unbeknownst'. Second, the word feels literary and slightly old-fashioned. It belongs to storytelling and to writing — novels, news reports, history — much more than to a conversation about your day. It also carries a small flavour of drama: writers reach for it when the not-knowing is going to matter, when the person who was in the dark is going to be surprised, hurt, or betrayed later in the story. You would not normally say 'unbeknown to me, it was raining' about something trivial. Grammatically it works as an adjective, but a strange one: it will not go in front of a noun (there is no 'an unbeknown fact'), and it will not sit after 'be' the way most adjectives do. It appears with 'to' and nothing else.

在某人不知情的情況下、瞞著某人