What it means
なり attaches to a verb in its dictionary form and means that the next action or event happened immediately after the first one. It emphasizes an abrupt, nearly instantaneous sequence and is common in formal, literary, and written narration. The subject of both actions is usually the same, although context can allow a different subject. The construction describes a completed trigger followed by another event, so the following clause is commonly in the past tense.
Usage note
なり is a formal written construction and should not be confused with the repetitive pattern なり…なり, which means “whether…or” or lists alternatives. It does not normally have a separate polite counterpart; politeness is expressed elsewhere in the sentence.
Example sentences
駅に着くなり、彼は忘れ物に気づいて引き返した。
As soon as he arrived at the station, he realized he had forgotten something and turned back.
編集者は原稿を読み終えるなり、著者に修正点をメールで伝えた。
As soon as the editor finished reading the manuscript, they emailed the author the points that needed revision.