@sarah - re:...awkward sentence...
¡Hola sarah! I think I can understand your frustration with this lesson. I've been trying to master Spanish "de" (of) to show possession for what seems like weeks now.
As a native English speaker, the possessive form of him (his) is a nice compact way of showing possession. Apostrophe "s" and the possessive pronouns make the chore of expressing possession very easy in English.
However, Spanish doesn't have apostrophe "s" and the possessive pronouns (su and sus) lose information which may be known, such as the biological gender of the possessor. That's why they are considered a poor choice where the gender of the possessor is known.
It seems awkward to the native English speaker to have to rely on a proposition to show possession, at first. In fact this little sentence fragment is just a noun phrase. But it can seem as daunting as a whole sentence.
Give it some time though, and it will roll off your tongue like a native Spanish speaker in no time. :)
Spanish doesn't use possessive pronouns a lot. (his, her, its (without an apostrophe), their.) So the direct translation is "The dog of him." The spanish word "él" with an accent means him. (In spanish, "him" and "he" are the same word. As are "me" and "I," "us" and "we," etc.) While "el" with no accent means "the." "El perro..." "de él." "The dog..." "of him."
@aguang - re:..."su perro"...
¡Hola aguang! I believe duo does accept, "su perro" as a correct answer. I resisted learning how to use "de" (of) to show possession for as long as I could. But I eventually stopped using "su" if the gender of the possessor was known.
I still can't say that I have mastered "de", but it's worth making the effort to try. In this exercise sentence duo gives us the gender of the possessor and the spirit of the exercise is to help you mastered the use of "de".
Admittedly, it is a little tricky getting the order of what is being possessed by whom right as well as remembering to use the article of the object being possessed. But it will become natural before you know it. :)