"I like this dog."
Translation:Ich mag diesen Hund.
we put diesen here instead of dieser because hund is accusative object?
I see what you mean! When it comes to using this you either use diese (which Duo recommends as does my German friend) or you can just use the noun's article. Meaning: Der Hund can be that dog or this dog depending on context when spoken in German. Das Fenster - this/that window Die Hochzeit -this/that marriage
It could be if you literally translate it. But when you say: "Es gefällt mir." It is just a common way to say, "I like it.". Like a example, Facebook "like" button in Deutsch is called "Gefällt mir" actually. If you find it weird, you can also say: "Ich mag es.", both terms are accepted.