-(느)ㄴ다는 것이

Use it when you meant to do one thing but by mistake ended up doing something else.

인용·전달Quoting & reporting

Han: The intended action does not take past tense in this pattern; use 넣는다는 and mark the past only on the final verb.

Examples

소금을 넣는다는 것이 설탕을 넣고 말았어요.I meant to add salt but ended up adding sugar.

친구한테 문자를 보낸다는 게 엄마한테 보내 버렸어요.I meant to text my friend, but I sent it to my mom.

아빠한테 전화한다는 게 형한테 전화했어요.I meant to call my dad, but I called my older brother.

2호선을 탄다는 게 3호선을 탔어요.I meant to take Line 2 but got on Line 3.

-(느)ㄴ다는 것이vs-(으)려고 했는데-(으)려고 했는데 just says the plan didn't work out, without implying you did something else by mistake: 일찍 자려고 했는데 못 잤어요.

V+는다는 것이 (after a consonant)

V+ㄴ다는 것이 (after a vowel or ㄹ, ㄹ drops)

spoken: -(느)ㄴ다는 게

넣다 → 넣는다는 것이, 보내다 → 보낸다는 것이, 만들다 → 만든다는 것이

Attach it to action verbs. The intended action stays in this present form (넣는다는, not 넣었다는); tense goes only on the final verb.
The second clause reports a mistake that already happened (-았/었어요, often with -아/어 버리다 or -고 말다). It cannot end with a command, suggestion or future plan.
Both clauses share the same subject, most often the speaker describing their own slip.
In everyday speech 것이 is usually shortened to 게: 보낸다는 게.

Wrong: ~~소금을 넣었다는 것이 설탕을 넣었어요.~~

Right: 소금을 넣는다는 것이 설탕을 넣었어요.

The intended action does not take past tense in this pattern; use 넣는다는 and mark the past only on the final verb.

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