Describes actively connecting or linking something.
When to use
device connection, internet setup, connecting people
The nuance
Used when the subject actively connects something. It is an active verb.
연결하다 (yeon-gyeol-ha-da, yeongyeolhada) · 연결되다 (yeon-gyeol-doe-da, yeongyeoldoeda)
Both 연결하다 (yeongyeolhada) and 연결되다 (yeongyeoldweda) mean 'to connect,' but 연결하다 describes actively connecting something, while 연결되다 indicates a passive or resulting state of being connected.

One lane per word — no nested boxes, just the gist.
Describes actively connecting or linking something.
When to use
device connection, internet setup, connecting people
The nuance
Used when the subject actively connects something. It is an active verb.
Indicates a passive state or result of being connected.
When to use
device auto connection, phone connection status, network connection completed
The nuance
Used when the subject is the thing being connected or when connection happens passively or by others. It is a passive verb.
컴퓨터를 인터넷에 연결하다.
keom-pyu-teo-reul in-teo-nes-e yeon-gyeol-ha-da., keom pyu teo reul in teo nes e yeon gyeol ha da.
The subject actively connects the computer to the internet, so 연결하다 is correct.
컴퓨터가 인터넷에 연결되다.
keom-pyu-teo-ga in-teo-nes-e yeon-gyeol-doe-da., keom pyu teo ga in teo nes e yeon gyeol doe da.
The computer is passively connected, so 연결되다 is correct.
나는 친구를 전화에 연결되다.
na-neun chin-gu-reul jeon-hwa-e yeon-gyeol-doe-da., na neun chin gu reul jeon hwa e yeon gyeol doe da.
The subject must actively connect, so 연결하다 should be used instead.
전화가 친구에게 연결하다.
jeon-hwa-ga chin-gu-e-ge yeon-gyeol-ha-da., jeon hwa ga chin gu e ge yeon gyeol ha da.
Phone is the thing being connected, so 연결되다 is correct.
인터넷 연결을 확인하다.
in-teo-nes yeon-gyeol-eul hwak-in-ha-da., in teo nes yeon gyeol eul hwak in ha da.
Actively checking connection status, so 연결하다 is correct.
네트워크가 자동으로 연결되다.
ne-teu-wo-keu-ga ja-dong-eu-ro yeon-gyeol-doe-da., ne teu wo keu ga ja dong eu ro yeon gyeol doe da.
Indicates passive automatic connection, so 연결되다 is correct.
나는 케이블을 TV에 연결되다.
na-neun ke-i-beul-eul tve yeon-gyeol-doe-da., na neun ke i beul eul tve yeon gyeol doe da.
The subject actively connects, so 연결하다 is correct.
케이블이 TV에 연결되다.
ke-i-beul-i tve yeon-gyeol-doe-da., ke i beul i tve yeon gyeol doe da.
The cable is passively connected, so 연결되다 is correct.
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