突然

Describes an event that happens unexpectedly and abruptly, often carrying a sense of shock or surprise. It emphasizes the lack of preparation or warning. While it can be used in casual conversation, it has a slightly more formal and objective tone than its common synonym "kyū-ni" (急に).

Contrast with "kyū-ni" (急に): "Kyū-ni" is more versatile and frequently used in daily life to describe both sudden events and a rapid increase in speed or intensity (e.g., "the temperature rose suddenly"). "Totsuzen," on the other hand, focuses more on the "unexpectedness" of the occurrence. For example, a sudden heart attack or an unexpected visit is more naturally described as "totsuzen" because the focus is on the surprise factor rather than the speed of the action.

Grammatically, it functions both as an adverb and a na-adjective. When used as an adjective (e.g., 突然な出来事 - a sudden event), it modifies a noun to describe the nature of the occurrence as being unforeseen. In formal writing or business contexts, "totsuzen" is preferred over "kyū-ni" to maintain a professional tone, especially when apologizing for an unexpected request or change in plans.