Used in negative sentences to emphasize that something is not the case at all; meaning 'not at all' or 'in the slightest'.
その説明は全然わからない。
I don't understand that explanation at all.
最近は運動していないから、体力が全然ない。
I haven't exercised recently, so I have no stamina at all.
彼の話は全然信じられない。
I can't believe his story at all.
Used (often in written or emphatic speech) to mean 'completely' or 'entirely', strengthening a positive or oppositional statement (e.g. 'completely opposed').
彼らの考えとは全然反対だ。
Their idea is completely opposite to ours.
計画は全然別ものだと分かった。
I realized the plan was something completely different.
Colloquially (especially among young speakers) used in affirmative sentences to mean 'very' or 'extremely' (e.g. 'totally fine').
明日でも全然いいよ。
Tomorrow is totally fine with me.
この映画、全然面白かったよ！
This movie was really interesting!
時間は全然足りるから、急がなくていいよ。
We have plenty of time, so there's no need to hurry.