于是
yūshì · HSK 4
as a result
➡️ 于 (at/on) + 是 (this) = at this point → thereupon
Literally 'at this', meaning 'at this point in the sequence'. Used to link cause and effect: X happened, 于是 Y followed. Think of it as the Chinese equivalent of 'and so' or 'whereupon'.
于 — two horizontal strokes 二 + vertical hook 亅
于 originated as a pictograph of breath or sound flowing outward, depicting exhalation through a passage. In oracle bone script it resembled a curved line showing motion or extension. Over time it became a grammatical preposition meaning 'at,' 'in,' or 'to,' losing its original concrete sense.
是 — sun 日 + 正 (correct/upright)
Originally a compound suggesting 'the sun 日 directly overhead'—straight, correct, true. From this concrete sense of 'right' or 'this very thing,' 是 came to mean 'this' as a demonstrative, and later the modern copula 'to be / is.' The 'correct' meaning survives in words like 是非 (right and wrong).