"A girl is falling."
I wrote "Knabino estas falas." Given that it's wrong, I'm led to believe "estas" is only used when . . . describing nouns?
Use the verb esti (estas) to say what something is (noun or adjective).
Mi estas viro. Sxi estas virino. Mi estas bona. Sxi estas malbona.
No need to use it to express continuous actions like in English. You could, but the other verb would flex (e.g.: Knabino estas falanta).
I'm sure this is a common problem for native english speakers. In many languages, like Esperanto and German, the present tense of a verb is the same as its present participle. That is to say that, "He walks" and "He is walking" would be the same, "Li promenas". You are semi-correct in saying that "estas" is used for descriptions. I just started learning Esperanto recently, but I believe that there is a different word for describing feelings/states of being, as in "I am feeling good". Can someone confirm this for me?
So the present participle is the same as the present? If so, great! I would miss that very much from the German tree.
I believe it's safe to assume that - English seems to be less common in that it separates "is ???ing" from "???s." And considering Esperanto is supposed to be a simplified language, I think that furthers the evidence to support that your statement is correct. I'm a learner too, so I don't know 100%, but it sure looks that way!