immediately; instantly; right away
Describes doing something without delay, often in response to a situation. Similar to すぐに but more literary.
suddenly; abruptly; all of a sudden
Emphasizes the unexpectedness of an action, often with a sense of surprise. More dramatic than 突然.
He immediately stood up and left the room.
Suddenly, it started raining.
突然 is the standard word for 'suddenly'; やにわに carries a stronger nuance of abruptness and is less common.
Rare kanji form; the kana spelling is strongly preferred.
Rarely used kanji form; the kana spelling is strongly preferred.
The exact derivation is uncertain; the kanji 矢庭 (arrow garden) may be ateji, and the word likely originated from an older adverbial phrase.