Underdone describes food that hasn't finished cooking. Sometimes food is underdone on purpose.
Undergrowth is a word used to describe plants that grow underneath larger plants and trees in a forest or jungle.
Underneath is a more interesting way of saying under. The cat in the picture is underneath a coffee table.
An undershirt, also called a singlet or vest in some countries, is a garment you wear under other clothes as an extra layer. Undershirts are usually white and have no sleeves. An undershirt worn as outerwear is usually called a tank top.
A person or animal is underweight if they weigh less than they should. When you are underweight you can easily see your bones.
You are undressed when you are wearing either no clothes or you are not wearing enough clothes. The people in the picture are undressed. When you are taking your clothes off, you are said to be getting undressed. The opposite of undressed is dressed.
Something is unfinished if it has not been completed yet. A similar word to unfinished is incomplete.The opposite of unfinished is finished.
Someone is unfit if they are in poor physical condition, and get tired very easily when they do physical activity. The man in the photo is very unfit. Unfit also means something that doesn't suit a purpose, for example a rotten fruit is unfit to eat.
Unfortunate is another word for unlucky. Unfortunate means that something bad has happened that is out of your control. The opposite of unfortunate is fortunate.