Can fina and trevliga be fine?
No, it sounds quite non-idiomatic to refer to clothes as trevliga.
What about vackra kläder?
Would "He has nice clothes" be "Han har fina kläder"?
I tried vackra kläder and it was wrong!?
"Beautiful" and "nice" aren't quite the same are they?
I used "snälla" and it wasn't accepted. "Nice" is a very vague word. What are the ranges of use for fin, snäll, trevlig, vackar, etc.?
"Snäll" is nice in the sense of good natured, the opposite of mean. Not in the sense of looking good.
Nice betyder trevligt, och det blev inte accepterat