入り込む

1. Origins, structure, and basic meaning

• Structure: 入り込む (はいりこむ) is a godan intransitive verb for entering through an opening and penetrating well inside.
• Basic usage: 虫が部屋に入り込んだ reports a small insect entering through a window gap, while 砂が靴の中まで入り込んでいた describes fine sand having penetrated deep into a shoe.

2. Common pitfalls and similar expressions

• Completed entry and persistent result
  • 入り込んだ is a completed entry event, whereas 入り込んでいた is the persistent state after entry. Neither is a transitive action of placing an object inside.