Three Suffixes, One Rule
All three attach to the ます stem and turn one verb into another. Learning them together is efficient because the attachment rule is identical.
| Suffix | Adds | Example |
|---|---|---|
| 込む | into, inward, thoroughly | 飛び込む (jump in) |
| 合う | mutually, with each other | 話し合う (discuss) |
| 直す | again, properly | やり… see below |
込む — Inward or Thorough
The core is movement into something: 乗り込む (board), 詰め込む (cram in), 申し込む (apply).
It also means doing something deeply or persistently: 考え込む (brood), 黙り込む (fall silent).
合う — Reciprocal
Both parties do it to each other: 助け合う (help one another), 知り合う (get to know each other), 愛し合う (love each other).
Because it is reciprocal, the subject is usually plural, and the partner takes と.
直す — Do It Over
Redo the action, usually because the first attempt was unsatisfactory: 書き直す (rewrite), やり直す (start over), 見直す (review, or reassess someone favourably).
Do not confuse it with the independent verb 直す, which means to fix or repair.