Vocabulary lists teach you WHAT idioms mean; this point teaches you HOW they behave grammatically — because idiomatic expressions in French routinely break, freeze, or bend the rules you've internalized for the regular language, and a C1 learner needs to recognize the pattern of breakage, not just memorize each phrase in isolation. The single most important fact about idioms is that they are frozen constructions: the words, their order, their articles, and their prepositions are locked in place by convention, not by logic, and swapping in a 'more correct' word or a synonym usually produces something a native speaker won't even recognize as the intended idiom.
The first systematic way idioms break normal grammar is through fixed, often unpredictable prepositions that don't follow any productive rule. Compare avoir besoin de (to need), avoir peur de (to be afraid of), and avoir envie de (to want/feel like) — all use de, which might tempt you to assume avoir + emotion/state always takes de. But avoir confiance en (to have confidence in) takes en, tomber amoureux de takes de while tomber sous le charme de also takes de but tomber dans les pommes (to faint) takes dans with no possible substitution — you cannot say tomber dans le sommeil to mean 'to faint,' because dans les pommes is not compositional; the words don't individually mean anything relevant to fainting. The preposition is simply part of the idiom's fixed shell, memorized as a unit, exactly the way English speakers just know it's 'afraid OF' but 'angry AT' without deriving it from a rule.
The second way idioms break normal grammar is through non-literal or suspended agreement and gender rules. In avoir raison / avoir tort (to be right/wrong), raison and tort never take an article and never pluralize even when several people share the state: ils ont raison (not 'les raisons'). In faire attention (to pay attention), attention stays invariable regardless of how much attention is being paid. Past participle agreement, normally sensitive to preceding direct objects, is frequently suspended or fossilized in idiomatic verbal expressions: in se rendre compte (to realize), compte is a frozen masculine noun that never agrees with the subject even though se rendre otherwise would trigger reflexive agreement — elle s'est rendu compte, not rendue, because compte is treated as a direct object following the verb, blocking agreement, and this exception is idiom-specific, not a general rule you can extend elsewhere.
The third way is register-dependent doubling, where French has multiple idioms for the same literal idea that are not interchangeable because each belongs to a different register, and using the wrong one in the wrong context is a fluency tell even when every individual word is 'correct.' Casser sa pipe (to kick the bucket) is casual and slightly jokey; s'éteindre (to pass away, literally 'to extinguish oneself') is soutenu and appropriate for an obituary; décéder is administrative/legal register. None of these are literal translations of 'to die,' and none of them can be swapped for mourir without changing the tone of the whole sentence, even though they're often taught as synonyms. Finally, idioms frequently lock verb tense or mood in ways ordinary verbs don't: qui vivra verra (whoever lives will see / time will tell) is frozen in the future tense and cannot be rendered in the present or past without destroying the idiom, and à bon entendeur, salut similarly cannot be reconjugated or reworded — recognizing which parts of an idiom are truly frozen (immovable) versus which parts flex normally (subject, tense of a surrounding auxiliary) is the real skill this point is teaching.
Exemples
Elle s'est rendu compte de son erreur trop tard.
She realized her mistake too late.
rendu stays masculine/invariable — compte blocks the normal reflexive agreement with elle
Ils ont raison de s'inquiéter, mais ils ont tort de paniquer.
They are right to worry, but they are wrong to panic.
raison/tort never take an article, never pluralize, no matter the subject
Je suis tombée dans les pommes en pleine réunion.
I fainted right in the middle of the meeting.
dans les pommes is fixed; no literal substitute for 'pommes' works
Il faut faire attention à ce que tu dis devant elle.
You need to be careful about what you say in front of her.
attention is invariable here, unlike a normal noun modified by degree
Son grand-père s'est éteint paisiblement la semaine dernière.
Her grandfather passed away peacefully last week.
soutenu register for death — appropriate in an obituary, not in casual speech
Mon collègue a cassé sa pipe la semaine dernière, à ce qu'on m'a dit.
My colleague kicked the bucket last week, from what I heard.
casual/jokey register — inappropriate at a funeral, unlike s'éteindre
J'ai une confiance absolue en elle, jamais en lui.
I have absolute confidence in her, never in him.
avoir confiance takes en, breaking the de-pattern of avoir besoin/peur/envie
Qui vivra verra ce que l'avenir nous réserve.
Whoever lives will see what the future holds for us.
frozen in the future tense; cannot be said in present or past
Il pleut des cordes depuis ce matin.
It's raining cats and dogs since this morning.
des cordes is fixed; you cannot substitute another plural noun for the intended meaning
Elle a pris la mouche pour un rien.
She took offense over nothing.
prendre la mouche is entirely non-compositional — 'to take the fly' means nothing literal here
Erreurs fréquentes
- Swapping the fixed preposition for a 'logical' one, e.g. saying avoir confiance de instead of avoir confiance en, by over-applying the de-pattern from avoir besoin/peur/envie.
- Adding agreement to a frozen participle inside an idiom, e.g. writing elle s'est rendue compte instead of the correct elle s'est rendu compte, by treating compte as an ordinary agreeing object.
- Adding an article or plural to invariable idiomatic nouns, e.g. saying elle a la raison or ils ont les raisons instead of the article-free elle a raison / ils ont raison.
- Translating idioms word-for-word from English or building 'logical' French equivalents (e.g. inventing tomber dans le sommeil for 'to faint') instead of using the fixed, non-compositional French form.
- Using a register-mismatched idiom, such as casser sa pipe in a formal eulogy or décéder in casual conversation with friends, because all three 'mean the same thing' in a dictionary but not in real usage.
Pourquoi ça trompe les anglophones
- English idioms also have fixed prepositions and frozen structure, but learners rarely transfer that intuition to French — instead they treat French idioms as decomposable phrases and try to apply general grammar rules (agreement, article usage, preposition logic) that idioms are specifically exempt from.
- Textbooks and vocabulary apps usually teach idioms as flashcard-style meaning equivalences ('tomber dans les pommes = to faint') without flagging which grammatical rules are suspended, so learners don't know an idiom is 'frozen' until they get it wrong.
- Register-dependent synonyms (casser sa pipe vs. s'éteindre vs. décéder) are invisible in a bilingual dictionary, which lists them as equal translations of 'to die,' so learners have no cue that using the wrong one signals wrong tone rather than wrong word.