～に～がある・
Use this pattern to begin with a location and then say what or who is present there. に marks the location, while が marks the person or thing being introduced.
Start with the place
The place normally comes first because it sets the scene. The phrase marked by が then gives the new answer: the thing, person, or animal found there.
Things and living beings
Choose ある for an inanimate thing and いる for a person or animal.
Questions and short answers
The location is omitted from the answer because it is already clear from the question. Japanese often leaves out repeated information in this way.
Place first or topic first?
Use the pattern on this page when the place is established and you want to introduce what is there. If a particular person or thing is already known and you want to tell where it is, make that person or thing the topic with は and put it first instead. This is an existence pattern, not a general way to mark where an event happens. For a concert, lesson, or meeting, mark the event location with で rather than the に used here for the place where someone or something exists.