"Outside the sun is shining."
Translation:Ekstere la suno brilas.
It would be sunbrilo.
Putting the o in or not (sunobrilo) doesn't change the meaning but would be done if the result was easier to pronounce.
"Suna brilo" would be shine that is sunny.
You might be at the point where you would enjoy reading Being Colloquial in Esperanto cover to cover. This will answer all sorts of questions that you seem to be asking. There's also a version on line.