-(으)되

-(으)되 states or allows a first action and then adds a condition or limit to it, in a formal or written style.

대조·양보Contrast & concession

Han: This is a casual statement of two contrasting facts with no condition or limit, so -(으)되 sounds bookish and unnatural; -지만 fits.

Examples

술을 마시되 운전은 하지 마십시오.You may drink, but do not drive.

운동은 매일 하되 무리하지 않는 것이 좋습니다.Exercise every day, but it is better not to overdo it.

자유롭게 의견을 말하되 다른 사람을 비난해서는 안 됩니다.Feel free to share your opinions, but you must not criticize other people.

회의실을 사용하되 사용 후에는 꼭 정리해 주십시오.You may use the meeting room, but please be sure to tidy it up afterwards.

V (consonant stem)+으되

V (vowel or ㄹ stem)+되

N+이되

먹다 → 먹으되, 가다 → 가되, 만들다 → 만들되

It is formal or written: typical in rules, notices, instructions and advice. In casual everyday talk it sounds stiff, so -지만 is used instead.
The second clause usually carries the restriction, so it is often a command, request, prohibition, or -아야 하다 / -는 것이 좋다.
Stems ending in ㄹ take 되 directly without 으: 만들되, 살되 (not 만들으되).

Wrong: ~~이 식당은 비싸되 맛있어요.~~

Right: 이 식당은 비싸지만 맛있어요.

This is a casual statement of two contrasting facts with no condition or limit, so -(으)되 sounds bookish and unnatural; -지만 fits.

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