于 yú is a function word, a grammatical preposition rather than a picture of a thing. Its oracle-bone and bronze forms are usually read as a drawing of breath or sound carrying outward, and from very early on the character was borrowed to do purely grammatical work. Whatever the original image, 于 in classical and modern Chinese is essentially a relation-marker: it sits before a noun and says how the verb connects to it, in space, in time, in direction, or in degree. It is filed under the radical 二 èr (two) by stroke convention, not by meaning.

There is a second glyph to know. The traditional and classical texts often write this preposition as 於 yú, a separate character that overlapped 于 in many of its uses. In the simplified script the two were largely merged, so mainland writing uses 于 across the board, while 於 survives in traditional typesetting and in a handful of fixed expressions. Treat them as one word with two written shapes.

The senses that matter to a learner all flow from one idea, "the point a thing is located or measured against":

1. At, in, on (place and time). 于 + a location or a date: 生于北京 shēng yú Běijīng (born in Beijing), 始于 shǐ yú (to begin at), 位于 wèiyú (to be located at).
2. To, toward, for, about. The thing the action is directed at or concerned with: 对于 duìyú (regarding), 关于 guānyú (about), 属于 shǔyú (to belong to).
3. Than (in comparison). Placed after an adjective to mark the standard: 大于 dàyú (greater than), 高于 gāoyú (higher than), 优于 yōuyú (superior to).
4. Bound into disyllables. Many common two-syllable words end in 于 as a fossilized tail: 由于 yóuyú (because of), 终于 zhōngyú (finally), 等于 děngyú (equal to), 至于 zhìyú (as for).

The single thread is anchoring. 于 fixes a verb or a quality to a reference point: a place, a moment, a target, or a measuring line. Once you hear it as "with respect to," every use falls into place.

The most classical job of 于 is to plant a verb at a place or a moment. After the verb, 于 introduces the location or the time: 出生于上海 chūshēng yú Shànghǎi (born in Shanghai), 成立于去年 chénglì yú qùnián (founded last year). This is the formal, written counterpart of the spoken locative 在 zài. In speech you would say 他住在北京 (he lives in Beijing); in a biography or a formal notice you meet the tighter 生于北京, 卒于南京 (born in Beijing, died in Nanjing). The 于 frame is compact and bookish, which is exactly why it dominates formal registers.

Placed after an adjective, 于 marks the standard a thing is measured against, giving a tidy "greater than" frame: 大于 dàyú (greater than), 高于 gāoyú (higher than), 多于 duōyú (more than), 优于 yōuyú (better than). 三大于二 "three is greater than two" is how a Chinese maths text reads the symbol >. This is the formal cousin of the spoken 比 bǐ comparison (三比二大); 于 packs the same relation into a single bound word.

The other place a beginner constantly meets 于 is as the frozen tail of a common disyllable, where it no longer carries a separable meaning of its own. 由于 yóuyú (because of) opens a reason; 终于 zhōngyú (finally, at last) caps a long wait; 等于 děngyú (to equal) states an equation; 至于 zhìyú (as for) shifts to a new topic. You do not parse the 于 in these, you learn each as a unit. Recognizing that the same little particle is hiding inside all of them is what makes the list feel like one family rather than four unrelated words.

Hear 于 as a single English phrase: "with respect to." It never travels alone; it always points at a reference point sitting just after it. Born with respect to a place is 生于; concerned with respect to a topic is 关于; greater with respect to a number is 大于; arriving with respect to the end is 终于.

So whenever 于 appears, look immediately to its right and ask what reference point it is anchoring the verb to: a location, a moment, a target, a measuring line, or the end of a long wait. Name that anchor and the meaning of 于 is already in hand. And remember its written twin 於, the same word wearing a fuller coat in traditional and classical texts.