You've learned ます, ない, たい, れる, させる, right? They all share a common name: auxiliary verbs (助動詞).
Most people memorize them as scattered grammar points, but they're actually the core skeleton of Japanese grammar — what comes after a verb and how it changes is all governed by auxiliary verb rules.
Understand the categories and conjugation of auxiliary verbs, and you'll find Japanese grammar suddenly becomes systematic.
Two Fundamental Properties
1. They can't stand alone
Auxiliary verbs must attach to other words. You can't just say ます or ない by themselves — they only have meaning when attached to a verb:
- 行く → 行きます
- 食べる → 食べない
2. They conjugate
All auxiliary verbs change form. Some have many forms (6 types), others fewer (1-2), but all follow fixed rules.
Five Categories
Auxiliary verbs are classified into five types by their conjugation pattern. Know the category, and you know how it changes.
1. Verb-type
Conjugates like a verb with 6 different forms.
| Auxiliary | Meaning | Example |
|---|---|---|
| れる・られる | Passive/potential/spontaneous/honorific | 読まれる, 食べられる |
| せる・させる | Causative (make someone do) | 書かせる, 食べさせる |
These four are the "celebrities" of Japanese auxiliary verbs — the most uses and the most tested on exams.
2. Adjective-type (i-adjective)
Ends in い and conjugates like an i-adjective.
| Auxiliary | Meaning | Example |
|---|---|---|
| たい | Want to do | 食べたい, 行きたい |
| ない | Negative | 行かない, 食べない |
| らしい | Seems like | 学生らしい |
Conjugation example — たい:
- たい (base) → たくない (negative) → たかった (past)
Looks exactly like the adjective pattern 大きい → 大きくない → 大きかった, right?
3. Na-adjective-type
Conjugates like a na-adjective, producing forms like 〜な and 〜に.
| Auxiliary | Meaning | Example |
|---|---|---|
| そうだ | Looks like... | 美味しそうだ → 美味しそうな料理 |
| ようだ | Seems like... | 雨のようだ → 雨のような天気 |
4. Irregular-type
Conjugation doesn't fit any of the above categories — memorize individually.
| Auxiliary | Meaning | Special Feature |
|---|---|---|
| ます | Polite form | ます・ません・ました・ましょう |
| です | Judgment/polite | です・ではない・でした |
| だ | Assertion | だ・ではない・だった |
ます conjugation must be memorized separately: ます → ません → ました → ましょう. Looks irregular, but you use it so often it sticks naturally.
5. Non-conjugating type
Never changes — always looks the same.
| Auxiliary | Meaning | Example |
|---|---|---|
| う/よう | Volition/conjecture | 行こう, 食べよう |
Three Conjugation Rules
Auxiliary verbs don't just randomly attach to verbs — each auxiliary can only attach to a specific conjugation form of the verb.
Rule 1: Irrealis form (未然形) attachment
Auxiliaries that attach to the verb's irrealis form:
れる・られる, せる・させる, ない, ぬ, う
| Verb | Irrealis Form | + Auxiliary |
|---|---|---|
| 行く | 行か | 行かれる, 行かせる, 行かない |
| 食べる | 食べ | 食べられる, 食べさせる, 食べない |
Memory tip: Irrealis form connects to things that "haven't happened yet" — passive (done to), causative (make do), negative (not done), volition (want to do).
Rule 2: Continuative form (連用形) attachment
Auxiliaries that attach to the verb's continuative form:
ます, た, たい, そうだ
| Verb | Continuative Form | + Auxiliary |
|---|---|---|
| 行く | 行き | 行きます, 行きたい |
| 食べる | 食べ | 食べます, 食べたい |
Memory tip: Continuative form connects to things that "actually happen" — polite (ます), past (た), desire (たい).
Rule 3: Attributive form (連体形/dictionary form) attachment
Auxiliaries that attach to the verb's dictionary form:
ようだ, らしい
| Verb | Dictionary Form | + Auxiliary |
|---|---|---|
| 行く | 行く | 行くようだ, 行くらしい |
| 食べる | 食べる | 食べるようだ, 食べるらしい |
Auxiliary Verbs Can Stack
Auxiliaries can attach to each other, forming "chains." The key: each subsequent auxiliary attaches to the appropriate conjugation form of the previous one.
Let's break down a complex example:
食べさせられたくなかった (didn't want to be forced to eat)
| Layer | Component | Explanation |
|---|---|---|
| 1 | 食べ | Continuative form of 食べる |
| 2 | させ | Continuative form of causative させる |
| 3 | られ | Continuative form of passive られる |
| 4 | たく | Continuative form of desire たい |
| 5 | なかった | Past tense of negative ない |
Translation: (Back then) I didn't want to be forced to eat.
Looks complex? Each layer actually uses the same rules. Master the conjugation rules, and you can disassemble any chain, no matter how long.
Self-Test
Q1. 「行かせる」— what auxiliary is used? What verb form does it attach to?
Show answer
Causative auxiliary せる, attaching to the irrealis form 行か of 行く. Meaning: make (someone) go.
Q2. Break down each layer of 「読みたくなかった」.
Show answer
- 読み → continuative form of 読む
- たく → continuative form of desire たい
- なかった → past tense of negative ない
Meaning: (Back then) I didn't want to read.
Q3. Why do we say 食べられる and not 食べれる?
Show answer
食べる is an ichidan verb (下一段), so passive/potential requires られる, not れる. 食べれる is so-called "ra-nuki kotoba" (ら抜き言葉 — dropping ら), common in spoken Japanese but grammatically non-standard.
Summary
- Auxiliary verbs are the skeleton of Japanese grammar: verb + auxiliary verb + particle = the basic structure of Japanese sentences
- Five categories: verb-type, adjective-type, na-adjective-type, irregular-type, non-conjugating type
- Three conjugation rules: irrealis form (passive/causative/negative), continuative form (ます/た/たい), dictionary form (ようだ/らしい)
- Auxiliary verbs can chain together, with each layer following the same conjugation rules