〜合う "do to each other / together"

Reciprocal action between two or more people.

V-ます stem + 合う means "do to each other / do together": 話し合う (talk with each other, discuss), 助け合う (help each other), 知り合う (get to know each other). 合う on its own means "to fit / to match", so the compound shows that two or more people's actions fit together. The partner is marked with と, or the subject is plural (二人は, みんなで).

Talking about relationships and cooperation: discussions (話し合う), mutual support (助け合う), meeting people (知り合う, 付き合う = go out with / keep company with), and arranging to meet (待ち合わせる). Common in speeches about teamwork and in everyday stories about friends.

• V-ます stem + 合う: 話す → 話し合う, 助ける → 助け合う, 知る → 知り合う, 愛する → 愛し合う
• Partner with と: 友達と話し合う; or plural subject: 二人は助け合った
• Conjugates as Group 1: 合います / 合った / 合って
• Note: 間に合う (be in time) and 似合う (suit) are not reciprocal

1. 01

   友達と意見を話し合いました。

   ともだちと いけんを はなしあいました。

   I discussed opinions with my friend.

   話す → 話し合う: both sides talk; 友達と marks the partner.

2. 02

   困ったときは助け合いましょう。

   こまった ときは たすけあいましょう。

   Let's help each other when in trouble.

   助ける → 助け合う: mutual help, with ましょう as a call to cooperate.

3. 03

   二人は駅で待ち合わせた。

   ふたりは えきで まちあわせた。

   The two arranged to meet at the station.

   待ち合わせる (transitive 合わせる): arrange to meet; the two are the plural subject.

4. 04

   二人は大学で知り合って、三年後に結婚した。

   ふたりは だいがくで しりあって、さんねんごに けっこんした。

   The two met at university and married three years later.

   知り合う = get to know each other; plural subject 二人は.

5. 05

   問題があるときは、家族でよく話し合ったほうがいい。

   もんだいが あるときは、かぞくで よく はなしあったほうが いい。

   When there is a problem, it's better for the family to talk it over together.

   家族で (as a family) makes the group the joint subject.

• お互いに

  The adverb "each other" can add the same meaning to any verb (お互いに助ける). 〜合う builds the meaning into the verb itself.

• 一緒に + verb

  一緒に食べる = eat together (side by side). 〜合う means the action is directed at each other.

• Transitive / intransitive pairs

  合う (intransitive: match) / 合わせる (transitive: put together) is one such pair; 待ち合わせる uses the transitive side. Compare side by side

• ✗

  友達に話し合った。

  ✓

  友達と話し合った。

  Why

  The partner of a reciprocal action takes と (with), not に.

• ✗

  私は電車に間に合い合った。

  ✓

  私は電車に間に合った。

  Why

  間に合う (be in time) already contains 合う and is not reciprocal; do not add another 合う.

• ✗

  話す合う

  ✓

  話し合う

  Why

  合う attaches to the ます-stem.

Common compounds: 話し合う, 助け合う, 知り合う, 付き合う, 言い合う (argue), 笑い合う, 分かり合う (understand each other), 支え合う (support each other). The noun forms are common too: 話し合い (a discussion), 知り合い (an acquaintance), 付き合い (relationship / socialising). 待ち合わせ (meeting arrangement) is a noun made from the transitive 合わせる: 駅で待ち合わせをする.