Translation:But the wind is weak today.
I put this answer exactly and it was marked wrong. Was it because I used kanji instead of kana?
The accepted answer just sounds award. "However, it isn't windy" would sound more appropriate, but I guess that includes a negative and is less of a 1:1 translation.
I never understand the difference between the usage of the は and が particle. Any help?
It would help if actual English usage was adhered to, eg "light", not "weak".
I think only a few dialects of English use "light" wind instead of "weak". I'm American and I've never heard of a light wind, but it may be different in England or wherever you live. It seems Duolingo is quite strict with its word choice. Then again, there are quite some odd translations in other chapters, so I do agree.
Even in American English "light" is at least one hundred thousand times more common than "weak" for describing wind;