"The soldier eats rice."
Translation:El soldado come arroz.
This isn't comprehensive and I don't see soldado there, but it's helpful.
Duolingo should give partial credit for this and add in the translation area: "Did you know that blah blah is also correct? Click here for words that do not need masc/fem." They would then take the responsibility for NOT teaching this anywhere in their program and eliminating the frustration of the testee.
http://www.fundeu.es/recomendacion/la-soldado-femenino-apropiado/ written en Español, long story short, "El/La soldado" is proper Spanish, but "La soldada" won't result in a Supernova. Similar to "El/La Presidente"