Way back when I was taking real-life, in-person Japanese lessons, one of my teachers told me this:

"We can only use the word 全然 (zenzen) with negative verbs."

全然 (zenzen), by the way, is a word [an adverb?] meaning "(not) at all; wholly; totally; utterly; entirely; quite; completely; very; extremely."

Also, perhaps unsurprisingly, you can use it with positive verb... sometimes.

This is the version they teach in school:

全然 (zenzen) + NEGATIVE VERB
= not at all VERB // don't VERB at all

For example, here are some negative verbs...

運動しない。
undou shinai
I don't exercise.

食べたくない
tabetaku nai
I don't want to eat it.

知らない。
shiranai
I don't know.

勉強してない
bennkyou shite nai
I haven't studied.

Now, if we add 全然 to the front of those, it means "to not do those verbs AT ALL!"

全然（運動）しない。
zenzen (undou) shinai
I don't exercise at all.

(Note: I put the 運動 in parentheses, because I think this is most natural as the answer to a question like 運動する？ If it was already said in the question, then we don't need to say it in the answer.)

全然食べたくない。
zenzen tabetaku nai
I don't want to eat (that) at all.

(Note: This could either be because the person is full or because the food looks gross... which is why I put "that" in parentheses in the translation.)

全然知らない。
zenzen shiranai
I don't know anything about him.

(Note: This could also be "I don't know anything about that," or "I know absolutely nothing about that..." like most Japanese, it would depend on the context.)

全然勉強してない。
zenzen bennkyou shite nai
I haven't studied at all.

(Note: Here we have ～て（い）ない working as the negative present perfect! I write about how this works in this article.)

Mmm... Formulas!

This also works for past tense, by the way:

全然できなかった。
zenzen dekinakatta.
I just bombed that test.
(Literally: "totally + couldn't do.")

(Note: Yeah, I'm taking some liberties with that translation. The situation is, for example, students discussing a test just after class got out... they don't know the results yet, but the speaker is expressing that he/she was not able to answer any of the questions.)

And it works for the negative form of adjectives, too:

全然痛くない。
zenzen itakunai
I doesn't hurt at all.

(Note: For example, when you face-plant and want to cry, but instead you try to look tough in front of your crush.)

全然怖くない。
zenzen kowakunai
That's not scary at all.

(Note: For example, talking about a horror movie that you're watching with someone.)

Now let's get into the wonderful world of exceptions to lame rules...

★★　Usage #2 - Positive Verbs?!　★★

There are so many exceptions to the negative-only rule, that I don't know if it can even be called a rule.

For example, we can say...

全然違う。
zenzen chigau
You're totally wrong. // That's not it at all.

So we can use affirmative verbs too?!

Well kind of... I think the best explanation is that we can use "全然 + affirmative verb" only in cases where we are negating something... with an affirmative verb.

Huh?

For example, we just saw 違う (chigau), which means "to be wrong; to be different." Anytime you say this verb, you're negating what someone says, so it still works with 全然.

Similarly, you can say...

全然間違ってるよ。
zenzen machigatteru yo.
You're completely off.
(Literally: "totally + making a mistake + よ.")

The sense is negation.

This is why you'll often hear people use 全然 with affirmative adjectives and verbs when they are answering someone's question with an answer that is contrary to expectations.

Examples...

Ａ： 生魚食べれる？
Ｂ： 全然食べれるよ。

Ａ： namazakana tabereru?
Ｂ： zenzen tabereru yo.

Ａ： Can you eat raw fish?
Ｂ： Yeah, no problem.

(Literally, A: "raw fish + can eat?")
(Literally, B: "completely + raw fish + can eat + よ.")
(Note: The nuance is that Person A suspects that Person B can't eat raw fish, so Person B is negating their expectation... which makes 全然 still sound natural.)

That might sound confusing... so take solace in the fact that I learned this without being able to explain it... until I started teaching lessons like these.

A really common adjective you'll see used this way is いい（よ）, as in...

Ａ：遅くなってごめんね。
Ｂ：全然いいよ。

Ａ： osoku natte gomen ne.
Ｂ： zenzen ii yo.

Ａ： Sorry I'm late.
Ｂ： No worries.

(Literally, A: "late + became + sorry + ね.")
(Literally, B: "completely + good + よ.")

You might want to memorize both A and B here... they can come in handy when meeting up with friends!

★★　Usage #3 - 全然 + NOUN　★★

I first got the idea for this lesson last week when I visited my editor's office.

I was getting ready to leave, and he was looking up train times for me to see if I was going to be late for my train.

He said:

全然余裕。
zenzen yoyuu.
You have plenty of time.
(Literally: "totally + enough time.")

(Note: 余裕 can mean a lot of stuff, like "surplus, leeway, a margin, etc." But here it just means "enough time." Also, this could mean "You have plenty of time," or it could mean "I/We have plenty of time," depending on context.)

It struck me what a useful sentence that was--because it's SO simple!!

Then I got to thinking about a couple of other "全然 + NOUN" combos that I hear pretty much every single day...

全然大丈夫。
zenzen daijoubu.
I'm totally fine.
(Literally: "totally + OK.")

(Note: For example, if you fall down and someone asks 大丈夫？ You can respond with this. Like we saw with 全然いいよ, it can also be said to someone apologizing for being late.)

全然平気。
zenzen heiki.
I'm totally fine.
(Literally: "totally + fine.")

(Note: Pretty much used exactly the same way as 全然大丈夫.)

Two-word phrases like these are boss, because they make your Japanese sound really good... even though you're saying something very simple.