"au-dessous/dessus de" v "sous/sur"

Question

What is the rule for using "au-dessous/dessus de" rather than "sous/sur"?

Cécile Janty-Davies · Accepted Answer

Bonjour Harry, 
1. The key difference between "sur" and "au-dessus de" lies in contact and proximity:
Sur means "on" and implies direct contact or proximity. It's used when something is resting on, placed on, or very close to a surface:

Le livre est sur la table = The book is on the table
Il y a un oiseau sur le toit = There's a bird on the roof
Mets tes chaussures sur l'étagère != Put your shoes on the shelf!

Au-dessus de means "above" or "over"  and indicates something is positioned higher than something else, but without direct contact. There's space between the objects:

L'avion vole au-dessus de la ville = The plane flies above/over the city
La lampe est au-dessus de la table = The lamp is above the table
Les nuages sont au-dessus de nous = The clouds are above us

2. The difference between "sous" and "au-dessous de" mirrors the "sur" vs "au-dessus de" distinction:
Sous means "under" and implies proximity or direct contact underneath something:

Le chat dort sous la table = The cat sleeps under the table
Mes clés sont sous le journal = My keys are under the newspaper
Il fait frais sous les arbres = It's cool under the trees

Au-dessous de means "below" or "beneath" and indicates something is positioned lower than something else, but with more distance or space between them:

La cave est au-dessous de la cuisine = The cellar is below the kitchen
La vallée s'étend au-dessous de nous = The valley stretches below us
Son appartement est au-dessous du mien = His apartment is below mine

In English terms:

sous = under/underneath 
au-dessous de = below/beneath

Bonne continuation!