"You call the dog."
Translation:Galwch chi ar y ci.
Lots of Welsh verbs use an added preposition to modify their meaning, so in this case 'Galw' on its own means 'to call', while 'galw ar' is used to refer to the object or person being called. 'Galwch chi' is a short form of the second person plural which can either mean 'You call', in the present tense, or 'You will call' in the future.
Other examples of this construction in other places in the course = gwrando = to listen, gwrando ar = to listen to, dweud = to say, dweud wrth to say to (to tell).