"The girl is very happy."
Translation:La niña está muy feliz.
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:
It's an adverb. (Adjectives modify nouns, adverbs modify verbs/adjectives/other adverbs). "Muy" and other adverbs like it (demasiado, poco, etc) appear before the adjective or adverb they modify.
You are not alone! I have found it extremely frustrating. So I use google translate, as well as the DuoLingo discussions to give me perspective. Don’t get hung up on speed or points. And try tracking your mistakes or discoveries with pen and pad. Nothing is perfect. I would be interested to hear from the DL creators on this.
Girl can be translated into Spanish as niña/chica/muchacha, and in some places in deep Spain also say moza and mozuela. But señorita means young single woman.
You've got the two verbs backwards. "Estar" is used for states/conditions (it shares the same root as "state" in the English phrase "state of mind/being"). "Ser" on the other hand is used for essential characteristics (tall/fat/smart/etc).
You can think of "estar" being used for temporary things and "ser" for permanent, but that's not a steadfast rule. As you've noticed already, locations (even if permanent) are described using "estar".