Now the big question: What is the difference between
sous und dessous
Dessous is an adverb, sous is a preposition. The preposition sous is used before the object, which it defines in relation to another object. The adverb dessous describes a verb or another adverb (and not the relation between objects) in more detail.
Mettez le livre sous la table.
Put the book under the table.
Mettez le dessous.
Put it under it. / Put it underneath.
In English there is no difference, which makes things a bit difficult.
One thing is important: the partner in the conversation needs to know what is talked about. To give a hint, which of the object (that were mentioned before) is really meant, the construction can also be the following:
Qui se cache là-dessous?
What do you hide under it?
Well, the easiest way to get a clear picture is to have a look at the function of the little word. Sous is a preposition that describes the relation of two objects. Dessous is an adverb, it describes a verb or another adverb more in detail. Sometimes there is not even an object to which a relation can be created. This also applies for constructions with preposition and dessous.
Elle va dessous.
She goes down.
Nous sommes passés par-dessous la barrière.
We went under the bridge.
L'air passe par-dessous l'aile.
The air goes under the wing.
Another case is ci-dessous, it means 'hereunder' and is a pronominal adverb..
Les exemples ci-dessous illustrent ce que nous venons de dire.
The examples hereunder illustrate, what we wanted to show.
There is, however, one exception: if the object is not directly under the other object, but more below or beneath the other. Here dessous becomes a preposition functioning together with another preposition to express the relation between the objects.
La ville se trouve en dessous du niveau de la mer.
La ville se trouve au dessous du niveau de la mer.