于 (yú) is a literary preposition equivalent to 在 (zài, at/in) in formal and written Mandarin. It indicates time, place, or source (生于, born at; 于...时, at the time of; 源于, originating from). In modern speech 在 is used in most cases, but 于 remains common in formal writing, official documents, and fixed phrases. Many compound prepositions built on 于 survive in everyday use: 对于 (regarding), 由于 (due to), 关于 (about).