Below The climbers stopped 300 m, below the top of the mountain. A moment later the sun had disappeared below the horizon.
Measurements : below The temperature is three degrees below zero. Parts of Holland are below sea level. The plane came down below the clouds.
Under I think the cat is under the bed. What are you wearing under your sweater? The whole village is under water. We usually use under, not below , to mean less than or younger than. There were under twenty people at the game. You cann’t see this film if you are under 18.