She sings a song when she is happy.

Meaning: This means she expresses her happiness by singing.

Choice of Words: sing is a verb that describes the act of making music with your voice. song is a piece of music with words. when is a conjunction that indicates the time something happens. happy is an adjective that describes a state of being content or pleased. When you put them together, "sings a song when she is happy" means she expresses her happiness by singing.