全く～ない is a strong, emphatic way to say 'not at all'. It is used in both spoken and written Japanese, but in conversation it can sound quite forceful, so it is often used when you want to express complete denial or disagreement. For example, if someone asks if you understood a complicated explanation, you might say '全く分からない' to mean you have no understanding whatsoever.

This pattern can be used with verbs, adjectives, and nouns. With verbs, simply place 全く before the negative form: 全く見ない, 全く知らない. With い-adjectives, use the negative form: 全く良くない, 全く面白くない. With nouns and な-adjectives, use 全く～ではない or 全く～じゃない (casual): 全く興味がない, 全く関係がない. Note that 全く is often written in hiragana, but can also be in kanji as 全く.

Be careful not to use 全く with positive predicates, as it would sound unnatural. The negative must be present. Also, in very formal writing, you might see 全く～ない, but in everyday speech, 全然～ない is more common and less strong. 全く adds a touch of formality or intensity.