A clean room makes you happy.

Meaning: This sentence suggests that a clean room can make people feel happy. It's like saying, One specific room that is clean can cause you to feel good.

Choice of Words: A is used to refer to one specific thing. Clean means to remove dirt or make something tidy. Room is a space in a building that is enclosed by walls and usually has a door. Makes means to cause something to happen or exist. You refers to the person or people being spoken to. Happy means feeling or showing pleasure or contentment.