grammar
Le complément de nom: Linking nouns with "de" and "à".
Bonjour ! In English, we simply stack nouns together to create new concepts: "water bottle," "history teacher," or "wine glass." In French, you cannot place two nouns side by side. You must connect them with a preposition-most commonly de or à-to explain their relationship.
The Preposition « De » (Category, Subject, or Material)
You use de to describe what kind of thing the noun is, its subject matter, or what it is made of. The second noun simply defines the category of the first one.
un cours de piano (a piano class)
un professeur d'histoire (a history teacher - note the d' before a vowel)
un sac de voyage (a travel bag)
un jus de pomme (an apple juice)
The Preposition « À » (Purpose or Function)
You use à when the second noun explains the specific purpose, function, or mechanism of the first noun. It answers the question: "What is this object used for?" or "How does it operate?"
un verre à vin (a wine glass - a glass used for wine)
une brosse à dents (a toothbrush - a brush used for teeth)
une machine à laver (a washing machine - a machine for washing)
The distinction between de and à can sometimes change the entire meaning of an object! Think about a teacup:
« Une tasse à thé » is a teacup. It is designed for tea, even if it is currently empty sitting in the cupboard.
« Une tasse de thé » is a cup of tea. This describes the actual content you are drinking right now.