Loose is an adjective and means slack or relaxed.
Lose is a verb and means 'to stop having','to have no longer'.
1. Her hair was loose.
2. The belt is loose.
3. The button is loose.
4. Loose terrible plagues upon humanity.
1. I have my lost my pen.
2. We lost the battle but we won the war.
3. I lost the opportunity to spend a year abroad.