L'aéroport était fermé à cause du brouillard.
The airport was closed because of the fog.
Il y a souvent du brouillard le matin.
There is often fog in the morning.
Il était impossible de conduire dans le brouillard.
It was impossible to drive in the fog.
FrenchTextbook teaches rules like this, in a full beginner course.
162 lessons, 4,000+ exercises, and 6+ hours of audio.