"I am good today."
很 does not really mean "very." It's kind of just used to indicate that an adjective is next. There are more advanced ways to say "very" that actually mean "very," but using 很 before an adjective is just the standard basic grammar structure. It confused me too when I first learned it. I hope this helps.
Agree about modifier comment. 好 is good, 很好 is very good. The only thing I can say about this is that in my 2.5 years of college Chinese (a long time ago) I remember that culturally 很好 came pretty much as a stock phrase together, no one said 好without 很 in this context. Is this a grammar/semantics issue or a linguistic/cultural issue? I have never heard a Chinese person say, 我今天好. However, I have said 我好 before to Chinese colleagues (programmers, doctors, no professional grammarians), who knew I was trying to improve my Mandarin and they did not correct me.
That being said, 很 is very. It is used as very in other structures, so the commenter that said 很 is not very, is very wrong. It is not a connector. It is an adverb.
My Chinese dictionary says: 很 - (adverb of degree) quite, very, awfully.
Before you make linguistic claims you should check your assumptions with data before posting them, then post your resource. Now here is an smiley emoticon to take the sting out of my comment. :)
I've lived in Taiwan, and I've never heard any one day 'hen hao' instead of 'hao' in this situation. I understand that this is what Duolingo has decided is correct, but it seems to be arbitrary.
I would say anything, but it costs me hearts, which is frustrating to lose hearts do to these types of 'errors'.
I'm a beginner learner, but it seems words to indicate the time are typically put at or near the start of sentences and not the end. In this case, both wo jintian hun hao and jintian wo hun hao are ok, but not wo hun hao jintian.
Just like in English you can say today I'm fine and I'm fine today, but not I'm today fine or I today am fine. The order matters but does not follow the same rules in different languages.
Hope it helps!
Yes I would say it is wrong. Adverbs (and any other adverbial expressions describing the verb) normally come before the verb.
As Nyaeay and Dmitrievdv mention below, 很 works in this case as a connector between the subject and an adjective verb. Nevertheless,
"我今天好" should also be correct. If I am not wrong, that would mean something like "Today I am (indeed) good" (as opposed to other recent time when I was not so good). But still valid.