込む【こむ】

込む goes on a verb's ます-stem and makes a compound verb. It adds one of two meanings: movement into something, or doing the action deeply, until you are sunk in it.

The verb is in the ます-stem: ×飛ぶ込む. Each compound is learned as a word, since which meaning 込む adds depends on the verb.

Into

With verbs of moving, putting and carrying, 込む adds "into". The place it goes into takes に.

Deeply, for a long time

With verbs of thinking, talking, sitting or sleeping, 込む says the action goes deep or goes on until you are absorbed in it.

With おもう思う and しんじる信じる the belief becomes fixed, and often wrong: おもいこむ思い込む is "be convinced", "assume".

Usage notes

These go beyond the two meanings.

Compounds with a meaning of their own

Some compounds have moved away from either meaning. おちこむ落ち込む is "fall into", but more often "feel down"; まきこむ巻き込む is "drag someone into" a problem.

こむ on its own

As a verb on its own, こむ means "be crowded", and is usually written こむ混む.