"The girl is very happy."
Translation:La niña está muy feliz.
I wrote "la niña está feliz" - missing the word for "very", but my answer was accepted with a correction "You missed a space: La niña es_tan feliz." There's no way to report situations like this. "My answer should not be accepted"? :)
I read a rhyme on another Duolingo thread that said "For how you feel or where you are, don't forget to use estar." It has helped me a lot; maybe it will help you too.
The verb "estar" is used for temporary status and conditions. There's not a super-easy mnemonic for determining which verb (ser or estar) is correct ("to be dead" for example, uses estar), but that's a general rule of thumb.
Check out this page for a more comprehensive list: