-던데① vs -는데①: I Noticed, or I Am Setting Up Now

The bag in your hand

You pick up a bag at the door. You have not walked away from it.

가방이 무거운데요.
gabangi mugeoundeyo — It’s heavy, and…

는데 sets the scene you are in. Soft lead-in. The rest can stay unsaid: so I will take a cab, so you carry it. No memory trip.

Swap the ending.

가방이 무겁던데요.
gabangi mugeopdeondeyo — I noticed it was heavy.

던데 reports a noticing. You checked earlier. You are putting that find on the table, often so the other person can react. If the bag is still in your hand and you never left it, 무겁던데요 is the wrong first line. It is not a thoughtful 는데요.

Rain now, rain remembered

지금 비가 오는데 우산이 없어요 — live weather as background for the missing umbrella.

아까 보니까 비가 오던데요 — you noticed rain when you looked, and you are offering that notice. The listener can confirm, deny, or fetch a coat.

Do not turn every 는데 into 던데 to sound more Korean. Noticing needs a look you already took. A test tomorrow (내일 시험인데) is setup, not a souvenir.

One sense each, not a blend

Sejong splits both forms. This page is ① against ①. -는데① backgrounds. -던데① recalls a noticed past and often invites a response. -는데② and -던데② wait on their own pages.

A shop report makes the swap clean. Standing in the queue: 사람이 많은데요. Back at home: 사람이 많던데요. Same shop. The first is the room you are in. The second is the look you brought home.

The door works the same way. 지금 문이 닫혔는데요 is the live block. 아까 보니 문이 일찍 닫히던데요 is the notice you carried back. 날씨가 춥던데 코트를 가져갈까요? keeps -던데 as a link: I noticed the cold, so shall we take a coat? That still needs a look you already took. If you only remember one failed line, remember 무겁던데요 said while you never put the bag down. That is still 무거운데요 territory. Do not leave thinking both are fine if you add 요.