-고 들다: Pushing Into It — Rough, Insistent Doing

-고 들다 is not “enter.” As an auxiliary it marks doing the action roughly, pressingly, as if pushing.

Gloss: insisting on doing it — pushing, usually annoyingly. 우리말샘 들다 「028」: after a verb in the -고 들다 frame, do that action in a rough, pressing way. Collocations like 따지고 들다 are the classroom doorway.

What it does — attitude on the verb

괜히 따지고 들어요.
작은 실수에 트집 잡고 들었어요.
상대가 먼저 공격하고 들었어요.
그 문제는 너무 깊게 파고 들면 끝이 없어요.

The verb already names the act; -고 들다 paints how — insistent, often abrasive. Neutral “I did X” does not need it.

Forms

Do not confuse with main-verb 들다 (enter / cost / hold). If you can replace the line with “enter the room,” you have the wrong sense.

Neighbours

계속 떠들어 대요.     repetition (대다)
자꾸 따지고 들어요.   pressing manner (들다)
공부하기로 했어요.     decision

When to use — and when not

Use -고 들다 when the complaint is about someone’s pressing, quarrelsome way of doing the act — arguing, nitpicking, attacking.

Avoid it for neutral starts (공부를 시작하고 들었어요 as “I began”). Avoid using it as a synonym of -어 대다 just because both can sound annoying. Avoid calm meeting minutes — the auxiliary carries attitude.

Practice: take 따지다 alone, then add -고 들다, and feel the quarrel heat rise.