Translation:Mi casa tiene un techo azul.
Why is 'un' used in "my house has a blue roof" and 'el' in "his house has a grey roof"?
I think they just corrected their mistake. They are probably both accepted now.
Un techo - a ceiling/roof.
Mi casa tiene un techo azul - My house has a blue roof.
Su casa tiene un techo gris - His house has a grey roof.
In both cases "el" sounds more natural and "un" can be understood but it sounds strange.
That's what I thought but when I typed "Su casa tiene un techo gris" it maked me wrong and corrected it with "Su casa tiene 'el' techo gris", but "Mi casa tiene un techo azul" was correct. I can't figure why.
I'm not sure either, but then the Spanish rules on when you need to use the definite/indefinite article, the, a, an in front of a noun and when you don't is still a bit of a mystery to me!
Mi casa tiene un tejado azul suggests in Spanish that there are several roofs and one is blue. If we are describing the house and its only roof, we would say Mi casa tiene el tejado azul.
I don't know either, but maybe it's something like -- My house has a blue roof His is the house with the gray roof ?
sort of, but "tiene el tejado azul" would sound still more natural even in this case in which it is not impossible to say "tiene un tejado azul"
Tiene el techo gris always sounds more natural. But I suppose we could also say: Mi casa tiene un techo gris, cuatro paredes blancas, tres ventanas...
mi casa tiene un tejado azul es correcta. Roof es tejado, techo es ceiling. At least in Spain when we refer to the part outside we say tejado and techo is the part inside. We also can use techo with the meaning of tejado, but rarely. Anyway, they should admit "tejado" as translation of "roof" it is right and it is more normal.