When it comes to adjectives of beauty, there is a substantial continuum of descriptors, many of which will be strongly affected in speech by one's tone of voice. From "nice" (ho-hum) to "Nice!! (wow!)". Then you have: charming, pretty, lovely, beautiful, gorgeous. I would suggest that the thing to take away from this particular lesson is that "beau/belle" is at the higher end of the scale and use a comparatively higher end word in English to translate it; one that does not rely on an emphatic voice to convey the meaning.
I'm also wondering why pretty was not accepted here. It feels like a semantic swamp, since English is not my native language. I suppose, I don't really understand or know all the nuances between beatufull, pretty, nice, good looking etc. and then the same applies to French (which I used to know and am now relearning). Trying to figure out why one word can be used as a translation but not another is really frustrating, especially since I occasionally do have to run things through my native language. So I'd really appreciate it if someone would clarify this for me and for all the others too.
as in another post above, belle= beautiful whereas joli= pretty.
In English, "beautiful" is a more intense word, whereas "pretty" is more casual and not as intense. beautiful is not equivalent to pretty. beautiful = very very pretty. I think that is why joli is not accepted here. it does not have the same connotation as belle.
Just by the way, I am a native English speaker and as for the other words you mentioned... ~"good-looking" is used to connote a physical beauty or sexiness. you wouldn't use it to describe a piece of clothing. ~"nice" is a pretty casual word (you wouldn't say, "your wedding dress is nice and gorgeous." "gorgeous" is very intense, so pairing it with "nice" doesn't make sense)
You are French but speaking about how you would translate it into English. The point is only this, that one may certainly say "that's a pretty dress". It is natural and grammatically correct. There are many positive statements that one may use to describe the dress but in the process of learning the language, there is a need to understand that you don't just pick a word from a list of similar words and assume it is the best answer. Is it close? Yes, it is. But if it didn't matter, Duo would just teach you one word "belle" and it would serve for all the positive remarks you want to make. The rest of the story is that there are many terms with some subtle shades of meaning.
I have some French friends, and one tells me that pretty is a close secondary translation of belle and can be used. And, having checked myself, it seems that belle and pretty do seem to crop up 'holding hands' in a few Eng - Fre dictionaries. I may, of course be asking in the wrong places. Duolingo is a fine language-learning tool though.
Most adjectives follow the noun in French. Those that always precede it are those denoting beauty, age, number, goodness and size ("BANGS"). There are also some adjectives that swing both ways, and mean different things depending on whether they precede or follow the noun (e.g. brave, grand, ancien, ...)
When referring to clothing, "gorgeous" would be "magnifique" or "somptueux". http://www.larousse.fr/dictionnaires/anglais-francais/gorgeous/584099