Such verbs are rare in Japanese, where the active (i.e. 他動詞 (たどうし, transitive); doing [some action] to [something]) and the mediopassive (i.e. 自動詞 (じどうし, intransitive); having [some action] done to [oneself] or having [some event] happening to [oneself]) have often been separated into two (or more) similar, etymologically related but distinct verbs following Classical Japanese. A good knowledge of these pairs will essentially be learning two verbs for the price of one. This article will attempt to establish some rules to hopefully help one recognize whether a verb from a pair is active or mediopassive.

Below, the words 他動詞, active and transitive are taken to be synonyms and used interchangeably; same for 自動詞, mediopassive and intransitive. The notation e.g. “-e” is used to mean “all syllables ending in e” (i.e. えけせてねへめれ and their dakuten variants). Readings of all kanji appearing for the first time in this article are in parenthesis.

す vs る endings

To my knowledge, one of the only infallible rules is that in pairs where one verb ends in す and the other in る, the former is always active while the latter is mediopassive.

As a rule of thumb, verbs ending in す are active while longer (3 syllables or more) godan verbs (i.e. type I verbs / -u verbs) ending in る are mediopassive.

The following are a few common examples (some words here have many more meanings than those listed and some have many kanji writings; only one is listed for brevity):

All verbs on the intransitive column in the table above are godan (including 帰る despite it ending in -eる); this is no longer the case in the tables below where all verb conjugation classes are regular.

-eる vs -aる endings

The second infallible rule is that if the pair has verbs ending in -eる and -aる respectively (the root part must be the same), then the former is always active and the latter, mediopassive. The caveat, however, is that many verbs ending in -eる are mediopassive (see in particular the -eる vs -u endings section below as well as the -asu vs -eru table above), so this rule only really works in pairs.

-eる vs -u endings

For verb pairs where one verb ends in -eる and another in -u (which are numerous), no apparent regularity exists as to which is active and which is mediopassive. They are only listed here to help learning them and to illustrate this irregularity.

Note: 入る as いる is rare and usually only occur in set compounds; 入る as はいる is much more common for the sense of “to enter”. Both verbs are (irregularly) godan.

Conclusion

Of course, there are many more irregular verbs pairs that can only be memorized. The fact that in some pairs one form is much more common than the other also helps. Also, many verbs don’t have pairs so nothing here applies. Unless one recognizes a very similar looking verb (notably one with the same kanji with the same reading, but that had a different ending) that one has seen before, always assume that the verb does not belong to such pairs.

In case an educated guess is needed, I came up with the following rules, in that order (note that the verb in question must be not conjugated, i.e. in dictionary form):

• Ending in す: active;
• 3 syllables or more and either ending in る and godan, or ending in -iる: mediopassive;
• Ending in れる: probably mediopassive (notable exception is 入れる);
• Ending in -eる: try to recall or guess the verb it is paired with: the other verb being -aる guarantees that this one is active, while it being -aす guarantees the opposite.
• This should cover a significant portion of all verbs. Other verbs are usually either relatively common (and thus well known to be active or passive) or rare enough that they can be learned while ignoring the pairing.

Basic Logical and General Terms

English / Symbols	Japanese	Notes, Readings
Let	とする	lit. Make it so that
Consider	を考える	かんがえる
is said to be	を という
represents	を と表す	あらわす
is saying	は を言っている	いっている（いう）
We can write	と書ける	かける（かく）
	ならば	lit. If ,
	イコール と は等しい	ひとしい
	と は等しくない
	は よりも小さい	ちいさい
	は よりも大きい	おおきい
	は 以下	いか
	は 以上	いじょう
	たす
	ひく
	かける
	わる
	分の	ぶん
	対	たい
	の 乗	じょう
	ルート
	乗根	じょうこん
	プラスマイナス
	ダッシュ	lit. dash
	ツーダッシュ	lit. two dash
	が に含まれる	ふくまれる（ふくむ）
(logical or)	または
(logical and)	及び そして かつ	および
(logical not)	ノット の否定 でない	ひてい
(congruence)	と は合同	ごうどう
(equivalence)	と は同値	どうち
Because ,	ので	Like in English, there are many other ways to express causation
When ,	とき
Given	とおいた
We get	とおく
satisfies	が を満たす	みたす
Regarding/For/Concerning	に対して について
There exists such that	が存在して	そんざい
Some	ある
All	全ての	すべて
(The) entire	全体	ぜんたい
holds	が成り立つ	なりたつ
To give (example, answer, etc.)	を挙げる	あげる
Making use of	を用いて	もちいて（もちいる）
It turns out that	がわかる
In particular	特に	とくに
However	しかし ただし
Already	すでに
Now	さて
The following	次	つぎ
That is to say (i.e.)	すなわち
For example (e.g.)	例えば	たとえば（たとえる）
Any	任意の	にんい
Property	性質	せいしつ
Contradiction	矛盾	むじゅん
Example (situation or problem from e.g. a textbook)	例題	れいだい
Note	注意	ちゅうい
Problem/Question (from a textbook, test etc)	問	もん
Definition	定義	ていぎ
Proposition	命題	めいだい
Proof	証明	しょうめい
Lemma	補題	ほだい
Mathematics	数学	すうがく

Algebra (elementary & advanced)

English	Japanese	Notes, Readings
Algebra	代数（学）	だいすう（がく）
To solve (an equation)	を解く	とく
To look for , to solve for	を求める	もとめる
is solvable	が解ける	とける
Equation	方程式	ほうていしき
Solution (as in “way to solve”)	解法	かいほう
Root, solution (of an equation)	根	こん
Vector	ベクトル
Linear	線形	せんけい
Space	空間	くうかん
Semigroup	半群	はんぐん
Group	群	ぐん
Additive group	加法群	かほうぐん
Commutative group Abelian group	可換群 アーベル群	かかんぐん アーベルぐん
Element	元	げん
Inverse element	逆元	ぎゃくげん
Identity element	単位元	たんいげん
Associative	結合	けつごう
Law, rule	法則	ほうそく
Group of symmetries ()	対称群	たいしょうぐん
Natural numbers ()	自然数	しぜんすう
Integers ()	整数	せいすう
Rationals ()	有理数	ゆうりすう
Irrationals ()	無理数	むりすう
Reals ()	実数	じっすう
Complex numbers ()	複素数	ふくそすう
Quaternions	四元数	しげんすう
General linear group ()	一般線形群	いっぱんせんけいぐん
Special linear group ()	特殊線形群	とくしゅせんけいぐん
Ring	環	かん
Integral domain	整域	せいいき
Integrally closed domain	整閉整域	せいへいせいいき
Unique factorization domain (UFD)	一意分解環 一意分解整域 素元分解環	いちいぶんかいかん いちいぶんかいせいいき そげんぶんかいかん
Principal ideal domain (PID)	単項イデアル整域 主イデアル整域 主環	たんこうイデアルせいいき しゅイデアルせいいき しゅかん
Euclidean domain, Euclidean ring	ユークリッド環 ユークリッド整域	ユークリッドかん ユークリッドせいいき
Field	体	たい
Finite field, Galois field	有限体 ガロア体	ゆうげんたい ガロアたい