"We are selling our small dog."
Translation:Wir verkaufen unseren kleinen Hund.
Unseren: "Hund" is masculine akkusativ here (the direct object of "verkaufen")
Kleinen: take a look at Adjektivendungen (adjective endings): you can see it by giant tables of gender, number, case...or you can see by the thumb rules based on the preceding articles or pronouns. That tells us that with "unserEN" we must use "kleinEN"
Hope that helps
And (here)[http://www.nthuleen.com/teach/grammar/adjektivendungenexpl.html] you can find a great explanation on Adjektivendungen!
Learn German, they said. It's fun, they said. Here are your five billion ways to say "small." Get them wrong and you'll be unintelligible.
Same question here