"We feel very happy."
Translation:Nos sentimos muy felices.
Adjectives have to be plural if what they're describing is plural. Una persona se siente feliz. Dos personas se sienten felices.
Because feliz is used for one person and felices is used for several people
Think of "sentir" as feeling something, like an object. Here, we're talking about feelings or sentiments, which uses the reflexive verb "sentirse." Treat these as two distinct verbs with different meanings.
Why nosotros sentimos muy felices is not accepted? When do I use 'nos' and 'nosotros'?
Good question. "Nosotros" simply means "us" in English; other related forms would be yo, tú, ellos, etc. You could actually put that at the very beginning, but it's normally omitted because it's a bit redundant.
The verb here is "sentirse," and the conjugated form requires a specific reflexive as well; in this case, it's the "nos" placed before the "sentimos." Search on the reflexive forms for more info.