合（あ）う (au)
～合う — do ～ to each other; to fit, to suit
Pattern
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動詞ます形（語幹） ＋ 合うstem + 合う: 教える → 教え合う (teach each other) -
A は B に ＋ 合うA suits B: この色は服に合う
Doing something to each other
Verb stem + 合う shows an action that goes both ways or is shared by several people: 話し合う (talk it over), 助け合う (help each other), 出し合う (put in together). It needs two or more people, and they are marked with で or と.
To fit or suit
As an independent verb, 合う takes に and means that two things match: 服に合う色 (a color that suits the clothes), 体に合う (suits the body). Do not confuse the two: the stem + 合う combination has a mutual meaning, while the verb 合う alone is about fitness.
Examples
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クラスのみんなで意見（いけん）を出（だ）し合（あ）いました。
Kurasu no minna de iken o dashiaimashita.
The whole class exchanged opinions together.
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この色（いろ）はあなたの服（ふく）によく合（あ）いますね。
Kono iro wa anata no fuku ni yoku aimasu ne.
This color really suits your clothes.
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友（とも）だちと連絡先（れんらくさき）を教（おし）え合（あ）った。
Tomodachi to renrakusaki o oshieatta.
My friend and I exchanged contact information with each other.
Common mistakes
- Use the stem: 教え合う, not 教える合う.
- It needs two or more people. For one person acting alone, do not use ～合う.
More grammar points
- 上（あ）げる ～上げる — to finish ～ completely
- だらけ ～だらけ — covered in ～, full of ～ (something bad)
- がち ～がち — tends to ～ (usually undesirable)
- がたい ～がたい — hard to ～
- 気味（ぎみ） ～気味 — a touch of ～
- かける ～かける — half-done, in the middle of ～
- 切（き）れない ～切れない — cannot finish, cannot fully ～
- 切（き）る ～切る — do ～ completely, to the end
- 込（こ）む ～込む — into, crowded, deeply ～
- 向（む）け ～向け — intended for ～, aimed at ～
- 向（む）き ～向き — suited to ～, facing ～
- 直（なお）す ～直す — do ～ over again
- っぱなし ～っぱなし — leaving ～ as it is
- っぽい ～っぽい — ～-ish / tends to be ～
- たて ～たて — “freshly ～ed / just done”
- 的（てき） ～的 — “-like / -al” (makes an adjective)
- 通（とお）す ～通す — “do ～ all the way through”
- あまり あまりの～に — so ～ that ～
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