"The boy is just throwing a ball."
Translation:Мальчик просто бросает мяч.
Not a native and just learning myself (chiming in in hopes someone will answer)...
The way I read просто is "Sorry the boy broke your window, mistakes happen but he was simply/just throwing around a ball like boys do. Probably carelessly. We'll fix it and take it out of his allowance"
Where I see только as the boy was only throwing a ball. That action affected no one and nothing else (except for the annoying sound). He threw the ball, nothing else happened because of it. That was literally the only thing he was doing and the only action/result.
But I could totally be wrong.
Просто translates to just, simply; and только literally means "only, merely". Suppose the sentence is "sorry my boy broke your window, he was only playing". But in English "just" can be used here as well. In Russian только and просто can be used is this sentence the same way, it depends on where you want to make focus, on "just/simply", or on "only/merely".
Только tends to be used less in such a situation though and it's more often used for translating just "only". He was the only one who was throwing the ball - он был только одним, кто бросал мяч.