once

Indicates a single occurrence or event, often used to emphasize that something happened at one time but not at any other. It can refer to past events or actions.

• I visited Paris once when I was a child. — 私は子供の時に一度パリを訪れました。
• She only told me once that she loved me. — 彼女は私に一度だけ愛していると言いました。

Used to indicate that as soon as one event occurs, another event will follow; often used to introduce a dependent clause that describes the time an event happens.

• Once you finish your homework, we can go to the movies. — 宿題が終わったら、映画に行けます。
• I will call you once I arrive at the airport. — 空港に着いたら電話します。

Refers to a single instance or occurrence of something, often implying a specific event or experience.

• I've seen that movie on multiple occasions, but once was enough for me. — 私はその映画を何度も見ましたが、一度で十分でした。
• There was a time once when we lived in the city. — かつて私たちはその都市に住んでいた時代がありました。