Look at ない in these three sentences:
A. 行かない。 → I'm not going. B. 食べてはいけない。 → You must not eat. C. 仕方がない。 → There's no way around it.
All three ない look the same, but they have completely different identities. If you can't tell them apart, you'll make conjugation errors.
Identity 1: Auxiliary Verb — Negating a Verb
The most common ない, attached to verbs to express negation.
- 行かない → not go
- 食べない → not eat
- しない → not do
Key feature: This ない cannot be replaced with ありません.
- ✅ 行きません (use ません as the polite negative form)
- ❌ 行かありません (you cannot swap ない for ありません)
Conjugation Rules
| Verb Type | Dictionary Form | Negative |
|---|---|---|
| Godan | 書く | 書かない |
| Ichidan | 食べる | 食べない |
| カ変 | 来る | 来（こ）ない |
| サ変 | する | しない |
Identity 2: Subsidiary Verb — Negating a Complement
Attached after 「~て」 or other forms, functioning as a subsidiary verb.
- 食べてはいけない → must not eat
- 知らなくてはならない → must know
Key feature: This ない can be replaced with ありません.
- ✅ 食べてはいけません
- ✅ 知らなくてはなりません
Identity 3: Adjective — An Independent "Not Exist"
ない functions as an い-adjective on its own, meaning "there is not" or "does not exist."
- 仕方がない → There's no way around it
- お金がない → I have no money
- 時間がない → There's no time
Key feature: This ない can be replaced with ありません.
- ✅ お金がありません
- ✅ 時間がありません
How to Tell Them Apart
| Auxiliary Verb | Subsidiary Verb | Adjective | |
|---|---|---|---|
| Position | After verb 未然形 | After ~て, etc. | Used independently |
| Meaning | Negates action | Negates complement | Does not exist |
| Replaceable with ありません? | ❌ No | ✅ Yes | ✅ Yes |
| Example | 行かない | いけない | お金がない |
The simplest test: Try replacing it with ありません. If you can't, it's an auxiliary verb. If you can, it's a subsidiary verb or adjective.
Self-Test
Q1. What type of ない is in 「彼は来ない。」?
Show answer
Auxiliary verb. It's attached to the verb 「来る」 to negate the action. It cannot be replaced with ありません (you can't say 「来ありません」).
Q2. What type of ない is in 「やらなくてはならない。」?
Show answer
Subsidiary verb. It follows 「~なくては」 and can be replaced with ありません → やらなくてはなりません.
Summary
- ない has three identities: auxiliary verb (negates verbs), subsidiary verb, and adjective (does not exist)
- How to tell: If it can be replaced with ありません, it's a subsidiary verb or adjective; if not, it's an auxiliary verb
- The auxiliary verb ない attaches to the verb's 未然形 (irrealis/negative stem)