"The girl helps her brother."
Translation:La niña ayuda a su hermano.
Caught by the 'personal a' join the club!
This is one of the things I've really struggled to get to grips with and I still get caught out with it even now.
This explains it:
It drives me batty!
First, there might be an explanation on the desktop version. Second, although I agree that Duolingo shouldn't be the only way someone learns a language (nor should any non-immersive tool, for that matter), I will say that after doing Duolingo every day for over 4 years, I have developed an intuitive sense for the "personal a". I don't know how to explain it... I just kind of know it should be there. And I accept that there are weird things like this in any language (especially English) that need time and practice to master.
When the direct object (thing receiving the action) of the sentence is a person, the preposoition "a" is used. For example, in English: "David helps Jimmy." In Spanish: "David ayuda a Jimmy." Jimmy is the direct object in the sentence because he is receiving help and is a person. Therefore, we used "a" in our sentence. I hope this helps someone. I know it took me a while to figure it out.