全然(ぜんぜん)(Not) at all

Both ぜんぜん (with a negative verb) and 全く～ない mean 'not at all,' e.g. ぜんぜんできない and 全くできない both mean 'I can't do it at all.', with ぜんぜん being more casual in tone. In modern speech, both can occasionally appear with affirmative forms, e.g. ぜんぜん大丈夫 ('It's completely fine.'), though this was traditionally considered improper use.

Both ぜんぜん (with a negative form) and とても～ない can mean 'not at all', but differ slightly in nuance. とても～ない emphasizes impossibility, e.g. とてもできない and ぜんぜんできない both mean 'I absolutely can't do it.'. With adjectives like 美味しくない, where there is no sense of potential, 全然 is preferred, e.g. 全然美味しくない ('It's not tasty at all.').

ぜんぜん and すこしも～ない both emphasize negation and mean 'not at all.' A key difference is that ぜんぜん, especially in casual speech, can also appear in affirmative phrases, e.g. ぜんぜん大丈夫 ('It's totally fine.'). In contrast, すこしも～ない is more formal and strictly used in negative contexts, e.g. すこしも寒くない ('It's not even a little cold.').