A dog usually has an owner.

Use "a" or "an" when you first mention one thing that's countable (like "a dog" in Tom's example), but there aren't any other determiners (my dog, that dog, etc.) before the noun. Use "a" if the next word starts with a consonant sound like "dog." Use "an" if the next word starts with a vowel sound like "owner." After you've mentioned something specific for the first time, you can switch from "a/an" to "the": There is a dog over there. The dog must have an owner. Where is the dog's owner (the owner of the dog)? In Tom's example you can't switch to "the" because he's not talking about a specific dog, but you could make both nouns plural without changing the meaning: Dogs usually have owners.