"The woman eats a sandwich."
Translation:La mujer come un emparedado.
That's what I put just to see whether they would accept it but they didn"t. I've ordered a torta (sandwich) in Mexico many times.
I find that words that end with 'o'(diario,emparedado,libro,pollo) are all masculine, so you use 'un'. And words that end with 'a'(manzana) are all female, so you use 'una' There probably are exceptions, but for now I use this as a guideline.
because emparedado is masculine and takes un. Articles (el, la, un, una) must agree with the noun in number and gender