What this is
You have been reading au, aux, du and des since M1 without anyone telling you what they are. They are not four new words. They are two prepositions — à and de — crashing into two articles — le and les — and fusing on impact.
French refuses to let à le or de le stand next to each other. When the grammar of a sentence produces that sequence, the language rewrites it before it reaches the page. That rewrite is what this concept is about, and once you see it, a large amount of French you have been taking on faith turns into arithmetic.
The rule
Four fusions, and only four:
| Preposition + article | Becomes | Example |
|---|---|---|
| à + le | au | je vais au marché |
| à + les | aux | il parle aux enfants |
| de + le | du | je viens du bureau |
| de + les | des | le bruit des voitures |
And two non-fusions, which matter just as much:
| Preposition + article | Stays | Example |
|---|---|---|
| à + la | à la | elle va à la banque |
| à + l’ | à l’ | nous allons **à l’**église |
| de + la | de la | la clé de la maison |
| de + l’ | de l’ | le jardin **de l’**hôpital |
So the rule fires on two of the four definite articles and ignores the other two entirely. If you can see la or l’ after your preposition, there is nothing to do.
Why it works this way
Say à le out loud a few times and you will find your mouth doing the work for you. Two small unstressed vowels in a row, with nothing between them, is exactly the kind of sequence French has spent a thousand years flattening. Old French wrote al; that collapsed to au, and the spelling froze with a u that no longer corresponds to anything you pronounce. De le went the same way through del to du.
La survived because it has a consonant at the front. There is no vowel collision to resolve, so nothing happened to it. L’ survived because it had already done its own collapsing — it elided against the noun behind it and had nothing left to give.
This is also why aux sounds identical to au. The x is a spelling relic. In speech the only thing distinguishing aux enfants from au enfant is the liaison: aux enfants comes out o-zɑ̃fɑ̃, with a z that has nowhere else to come from.
Edge cases
There is no uncontracted version
This is the part English speakers treat as optional and it is not. À le marché is not informal, not old-fashioned, not regional. The string does not exist in French. Neither does de le. When you write one, you have not written a slightly wrong sentence — you have written something no native speaker has ever produced.
The same holds for à les and de les. If your grammar produces them, the contraction is mandatory.
Only le and les fuse
Half of the definite articles ignore this rule completely. À la boulangerie, de la maison, à l’école, de l’hôpital — all four stay exactly as written, two separate words. Learners who over-apply the rule start producing à l’ → al’ or reaching for au in front of feminine nouns. There is no contraction available there.
Note that l’ hides gender. À l’église and à l’hôpital look identical, and église is feminine while hôpital is masculine. You only find out when an adjective or a de + plural shows up.
des does two different jobs
Des livres means some books — that is the plural indefinite article from M1. La porte des maisons means the door of the houses — that is de + les, this concept’s contraction. Same three letters, unrelated grammar.
The way to tell them apart is to ask what is in front. If a de is being contributed by a preposition or a verb (parler de, venir de, près de), you are looking at the contraction. If nothing is, you are looking at the article.
The same split exists for du: du pain (some bread, partitive) versus du bureau (from the office, contraction).
What English speakers get wrong here
Translating the apostrophe-s. English says the boss’s office. French has no apostrophe-s at all and runs possession backwards through de: le bureau du patron, the office of-the boss. Every time you want a possessive with a noun, you are going to need this contraction.
Forgetting that verbs carry prepositions. Répondre à, parler de, téléphoner à, venir de — the preposition is part of the verb, and it hits the article just like any other. Il parle de le projet is wrong in exactly the way à le marché is wrong.
Confusing venir de + place with venir de + infinitive. Je viens du parc is motion — I’m coming from the park. Je viens de manger is the recent past from M10 — I’ve just eaten. What follows the de decides everything, and only in the first case does a contraction happen at all.
Contracting after a possessive. De mon frère, à ma sœur — possessives are not definite articles, and nothing fuses. The rule wants le or les specifically, not “any determiner”.