"That train is big."
Translation:Ese tren es grande.
'Eso' is used when you are not using the noun it refers to e.g. 'This is big', 'This is good'. Ese / esa is used when you name the thing e.g. 'this house is big', 'this book is good'
same here. you're supposed to use eso when you dont know the gender, which i took to mean objects, because you always know the gender of the article that precides a noun, since every noun has an el or la
Yes, but I don't know when to use 'ese' instead of 'eso' - aren't they both masculine?
Apparently not. "Ese" is masculine singular; "esos" is masculine plural. "Eso" is gender neutral.
"Eso" is gender neutral. You'd only use "eso" if you didn't know the gender of the noun being referenced, like if the sentence said "That is big." "That" would be unknown, so you'd use a gender neutral pronoun. Since we know the gender of "train" is masculine, we use the masculine pronoun "ese."