As if the myriad possible translations of à and de aren't enough, these two French prepositions also have complementary and contrasting uses.
Demonstrative adjectives (this, that) are used to indicate a specific noun or nouns. In French, they must agree with the noun(s) in number and sometimes gender: ce, cette, cet, ces.
For quantities, adjectives, and prepositional phrases, the question of de vs du, de la, des depends on whether the noun that follows is specific or unspecific.
When used as relative pronouns, qui doesn't necessarily mean "who" and que doesn't always mean "that"; depending on the context, either one can mean either one.
About two dozen intransitive French verbs require être as their auxiliary in the compound tenses and moods. Of these, eight can be used transitively, and when they are, two things happen.