"The sofa in my house is white."
Translation:El sofá en mi casa es blanco.
Argh that caught me too (I was debating which it was for a while)...for the most part, you can accept the rule that -a means feminine and -o means masculine (and -e goes either way) and just remember the exceptions. Like la mano (the hand).
Another thing to look at is how most nouns that start with a or ha will have el instead of la (I think originally because of how it sounded) so you'd have el agua, but las aguas.
Actually, I think it's non-gender specific, so the default is masculine.
When you hover over, it will tell you at the bottom M or F. Not having a gender means the default of masculine is used.
Similar to "Who are they?" being translated to "¿Quiénes son ellos?" "ellos" is used because the gender is unknown, and thus masculine is used by default.
Don't worry though, this one caught me as well. :/