"You have one dog."
Translation:Tha aon chù agad.
Ordinarily you'd use cù if you were talking about a dog being something, like if you wanted to say it's a big or sweet or brown dog.
If you say "one dog" or "two dogs" in English, you still use the singular version in Gaelic (cù). However, the words for one and two cause something called lenition - the h that gets inserted after the c. So it's still singular but the spelling changes because "aon" and "dà" do that to words.
If you wanted to say three (or give our ten or anything that's three or more) dogs, it uses the plural version of coin.
What does all of that look like?
"you have a dog" - "tha cù agad"
"you have one dog" - "tha aon chù agad"
"you have two dogs" - "tha dà chù agad"
"you have five dogs" - "tha còig coin agad"