근데 근데 is an "informal" contraction of 그런데.   그러나/하지만/그렇지만 그러나/하지만/그렇지만 is only used when the second clause negates what is said in the first. i.e. 점심을 먹었어요. 그러나/하지만 아직 배가 고파요. (I ate lunch. But I'm still hungry). 그러나/하지만/그렇지만 are very similar and are interchangeable in most cases. The difference is the nuance on how much of the first clause one is to acknowledge. For instance, 그러나 does not necessarily acknowledge the truth claim of the first clause when negating the idea. 하지만 acknowledge it; 그렇지만 (contr. 그렇다 하지만) explicitly states so. (For all intents and purposes, just go ahead and use them interchangeably unless you want to be super nit-picky on the grammar even for a native or an advanced speaker.) 하지만 also tends to be a bit more colloquial than and informal than 그러나 or 그렇지만.   그런데 그런데 is also used when contrasting the second clause from the former, but moreso as to mark change or shift in topic. i.e. 점심을 먹었어요. 그런데 집에 언제 가요? (I ate lunch. However, by the way, when do we go home?) Note: 점심을 먹었어요. 그런데 아직 배가 고파요 works as well.

There’s two very similar words in Korean, 속 and 안. Both nouns mean “inside”. But they’re slightly different.   Following examples are only possible with 안.   (1) 지하철 안 (inside of subway) *지하철 속 — Awkward (2) 교실 안 (inside of classroom.) *교실 속 — Awkward (3) 내 시야 안 (inside of sight) *내 시야 속 — Awkward   Following examples are only possible with 속. (1) 꿈속 (inside the dream) — Already one word that you have to write it without a space. *꿈 안 — Awkward (2) 물속 (inside of water) — Already one word *물 안 — Awkward (3) 돌 속 (inside of stone) *돌 안 — Awkward (4) 도넛 속 (inside of doughnut) *도넛 안 — Awkward (5) 달빛 속 (inside of moonlight) *달빛 안 — Awkward   And following examples are possible with both. (1) 신발 안, 신발 속 (inside of shoe) (2) 장갑 안, 장갑 속 (inside of glove) (3) 상자 안, 상자 속 (inside of box) (4) 입안, 입속 (inside of mouth) — All are each one word. (5) 주머니 안, 주머니 속 (inside of pocket) (6) 지갑 안, 지갑 속 (inside of wallet)   The antonym of 속 is 겉, and that of 안 is 밖. 겉 means “edge”, “crust”, “surface”, or “cover”, but 밖 means “outside”. Then, “non-겉” would be 속, and “non-밖” would be “안”. Put it in another way, 겉 and 속 is physical wheras 밖 and 안 is spatial.   (▲ Comparison between 밖 and 안) 밖: Outside of a particular range. 안: Inside of a particular range. (▲ Comparison between 겉 and 속) 겉: Crust, edge, or surface of a particular thing. 속: Inside of a particular thing other than its crust, edge, or surface.   Info