"They are very strong substances."
Translation:Son sustancias muy fuertes.
I think "ellos" should be accepted, but I'm not a native speaker, so ...
I did report it.
I believe it's "ellos" if the pronoun refers to substances of masculine (el acero) or unknown gender, and "ellas" if the pronoun refers to known substances of feminine (la piedra, la gema) gender.
"¿Por qué las gemas no tienen arañazos? Ellas son sustancias muy fuertes."
(Why don't the gems have scratches? They are very strong substances.)
Those two solutions that you spoke about have not been accepted in the past because those solutions were never added to the Duolingo database of solutions.
Either gender of subject pronoun (ellos, ellas) might feasibly be added to the Spanish sentence. The gender depends on which substance. The gender of the subject pronoun depends on the gender of whichever substance is being discussed.
In comparison with the default solution to the exercise, it is much easier to omit a subject pronoun instead of including one.
Can someone please tell me when do we use pronouns and when can we eliminate them. I seem to always choose the wrong way I put Ellos sustancias muy fuerte. Which is correct but I got dinged because I used the Ellos. Even if it is redundant it should still be correct. Right?