What it means
た瞬間に means “the moment” or “as soon as” an action is completed. It presents the event in the following clause as occurring immediately at the precise point when the preceding action or change takes place, often emphasizing suddenness or a direct temporal trigger. The preceding verb is normally in the た-form, and the construction is commonly used in written descriptions, narratives, and formal statements.
Usage note
た瞬間に describes an event that follows immediately, so it is not normally used for a long interval or a gradual consequence. Do not confuse it with 瞬間、which can function as a noun phrase meaning “the instant” and does not itself require the same verb attachment pattern.
How to use it
電車が駅に着いた瞬間に、雨が降り始めました。
The moment the train arrived at the station, it began to rain.
電車が駅に着いた瞬間に、雨が降り始めた。
The moment the train arrived at the station, it began to rain.