Mournful means very sad — but it is not the everyday word sad. It is the sadness of loss, of a heavy heart, of something that sounds like crying even when no one is crying. A face can be mournful. A song can be mournful. A cry in the night, a slow violin, an empty street in the rain — English likes to put this word on looks and sounds, not only on people. It is mourn plus -ful: full of mourning. You will meet it in stories more than in chat. If your friend failed a quiz, sad or upset is enough; mournful would make the quiz sound like a death, and that is too much. Though actually, writers sometimes stretch it a little, just to colour a scene, without a real funeral in sight — a dog can give you a mournful face when you leave the house.