The most alert anime-watchers probably have already figured nai means something negative. Pretty much every time nai is to be found something is not to be found. Example:
- shinjirareru 信じられる
- shinjirarernai 信じられない
- de aru である
- dewanai ではない
- iru いる
- inai いない
Is not here.
What happens is that, in Japanese, the nai suffix turns a verb in its negative form. This is difficult to notice if you rely only in subtitles and doesn't have some understanding of Japanese, because the translations are never literal (which is good) and that means that the "not" word doesn't necessarily show up in English when there is a nai in Japanese.
Literal Nai and Translated Nai
For example, shinjiru 信じる means "to believe." Technically, "to not believe" would be shinjinai 信じない, but nobody says that. Instead, they say shinjirarenai 信じられない, which is literally "can not believe." This rare られ comes from the potential form of the verb, which is shinjirareru 信じられる, "can believe."
Now, nobody is actually going to write some stupid phrase like "can believe" or "can't believe" in a translation, because scenes with these words usually carry a lot of emotion and it's more like "I TRUST U" or "OH MY GOD Y DID U DO DIZ 2 MEEeeee."
So even though nai always makes the verb negative in Japanese, you can only realize this if you understand Japanese. In the English translation that won't be reflected all the time, which it makes nai look like it has multiple meanings.
This specially shows up in the verb iru いる:
- iru いる
It's here. We got (the thing). We have it in store. Yep, here.
That person you looking for? Here. He's here.
- inai いない
It's gone. It disappeared. Where is (the thing)?!! WHERE IS IT?!?!?!
He's away. He left. Nobody's home.
(lit.: is not here.)
- iru 要る
It is needed. I need it. It's required.
- iranai 要らない
No, thanks. I'll pass. (when offered something)
(lit.: does not need.)
Nai with Desu Copula
One of the most common and impossible to notice uses of nai is with the desu copula. In English, our copulative words are "is," "are," and "be" (and their variations). So the negative of that would be isn't, aren't and not be. All of this achievable in Japanese with the desu nai combo.
Because there is no desu nai in Japanese! That doesn't exist. It's actually janai じゃない or dewanai ではない! You might not have noticed it, but every time you hear janai or dewanai in anime, it's actually desu and nai mixed together!
- neko da 猫だ
neko desu 猫です
It is a cat.
- neko janai 猫じゃない
neko dewanai 猫ではない
It is not a cat.
nai 無い - Does Not Exist
When the word nai is not used as a suffix (almost never happens) it's then an -i ～い adjective meaning... "nonexistent."
Now, if you are like me, an average Joe, I'm pretty sure this is the first time in the year you see the word "nonexistent" being used. This is because when something is "nonexistent" you don't say it is "nonexistent," you say something else. See:
- uchi dewa terebi ga nai 家ではテレビが無い
TV is nonexistent in my home. (robot way)
There's no TV in my home. (normal way)
- terebi no nai seikatsu テレビの無い生活
Nonexistent TV livelihood. (robot way)
Livelihood without a TV. (normal way)
The antonym of nai 無い is aru 在る. One is an adjective, the other is a verb. It makes no sense but it's true. See:
- teberi ga nai テレビが無い
There is no TV.
- terebi ga aru テレビが在る
There is TV.
You can only tell it in phrases like the above. Most of the time, aru ある is written without the kanji. One common word with the aru 在る, "to exist," kanji is sonzai 存在, "existance," which only servers to show how these two kanji (在 and 無) are pretty much antonyms.
Though I've been using the 無 kanji for nai in this part, the nai word like this is rare (the suffix is far more common) so I'd like to note that most of the time, if you see the 無 kanji written somewhere, it's probably not the word nai but something else.
Good thing, though, is that mu 無 (on'yomi reading) has a very strong meaning: it means "nonexistent." (ha! Who could guess!) So every time you see this kanji 無 you should know that something is "nonexistent," though it may not appear so at first glance. For example:
- muyou 無用
Useless. (no use)
- muryou 無料
Free. (no charge, no cost, no price)
- mugen 無限
Infinite. (no limit)
- muri 無理
Impossible. (no logic, no basis)
- mukou 無効
Ineffective. (no effect)
Nai With Other Suffixes
The way the Japanese language works allows you to put nai in any verb and almost any conjugation of any verb. Which means its meaning compounds with different conjugations.
For example, a verb in the present goes like this:
- taberu 食べる
- tabenai 食べない
Don't eat. Doesn't eat.
If it's potential or passive, it becomes like this:
- taberareru 食べられる
Is eaten. (passive)
Can be eaten. (potential)
- taberarenai 食べられない
Is not eaten. (passive negative)
Can't be eaten. (negative potential)
If it's causative, it goes like this:
- tabesaseru 食べさせる
To make (someone) eat (something). Force-feed.
- tabesasenai 食べさせない
To not make (someone) eat (something).
To not let (someone) eat (something).
If it has the -tai ～たい suffix, it becomes like this:
- tabetai 食べたい
Want to eat (something)
- tabetakunai 食べたくない
Don't want to eat (something)
So, basically, the nai suffix can be added to literally every verb in almost every of its conjugations, which is why it's such a frequent word.
One special note: the word shinai しない is the negative of the suru する verb. Every verb that ends with suru can be turned into negative by saying shinai instead of suru.
The word nai is technically an -i ～い adjective and as such it can be subjected to the way -i adjectives are conjugated.
For example, if samui 寒い means "cold", and samukatta 寒かった means "was cold," then tabenai, "doesn't eat" can become tabenakatta, "didn't eat."
This can be a little troubling to understand because in English we use the auxiliary verbs "don't" and "didn't" in order to create present negative and past negative forms. In Japanese, the nai suffix is adjective-like, and you conjugate it like an adjective to make that past negative form.
So, despite it looking inverted, tabenai is "don't eat" while tabenakatta is "didn't eat."
Verbs' Polite Conjugations
The word nai is normally used to make a verb negative, but sometimes it's not ideal. Outside of anime, in that thing called real life, it's often the case where a certain kind of speech, polite speech, should be used instead of casual speech.
In these cases, negative verbs tend not to end with nai but with masen instead. So, only when it's a verb suffix, nai is replaced by masen, and nakatta replaced by masen deshita in polite speech.
- tabemasen 食べません
- tabemasen deshita 食べませんでした
Although these have nothing to do with the word nai itself, they are actually conjugations of the masu suffix.