Causal conjunctions express the reason for a fact, an action, or a conclusion. They connect two clauses by indicating a cause-and-effect link, which places them at the heart of the Conjunctions and of the structures of Propositions and Subordinate Clauses. They allow choosing between a direct link, a justification, or a cause already known depending on the context and the register.

Because introduces the most direct cause and naturally answers the question why. This conjunction is very frequent in speech and suits the everyday register. It introduces a complete subordinate clause and is most often placed after the main clause.

IdéeIdea.	ExempleExample.
Cause directeDirect cause.	Il vient parce qu’il pleut.He is coming because it is raining.
Registre courantEveryday register.	Je pars tôt parce que je travaille.I’m leaving early because I have to work.
Position après la principalePosition after the main clause.	Nous restons à la maison parce qu’il fait froid.We stay at home because it is cold.

Car is a coordinating conjunction that links two clauses by giving a justification. Its use is more formal and more written than that of 'because', and it is rare in everyday conversation. It does not replace a cause placed at the head of the sentence, but adds an explanation after the main statement.

IdéeIdea.	ExempleExample.
Registre soutenuFormal register.	Il est absent, car il est malade.He is absent because he is sick.
JustificationJustification.	Je reste, car la réunion n’est pas finie.I am staying because the meeting is not finished.
Écrit formelFormal writing.	Le dossier a été accepté, car il était complet.The file was accepted because it was complete.

As placed at the beginning of the sentence signals a cause already known or evident. It introduces the subordinate clause before the main clause and thus creates an inversion of order compared to 'because' and 'since'. When it is at the head, a comma generally separates the causal subordinate clause from the main one.

IdéeIdea.	ExempleExample.
Cause en premierCause first.	Comme il pleuvait, nous sommes restés.As it was raining, we stayed.
VirguleComma.	Comme tu étais fatigué, tu t’es couché tôt.As you were tired, you went to bed early.
Cause connueKnown cause.	Comme le train avait du retard, nous avons attendu.As the train was late, we waited.

Since is used to justify a conclusion from a cause considered as known, evident, or already admitted by the interlocutors. It appears very often after the main clause, but it can also open the sentence in a more emphatic style. As with the other causal conjunctions, it introduces a complete subordinate clause and does not by itself impose the verb tense.

IdéeIdea.	ExempleExample.
Cause admiseAssumed cause.	Puisque tu sais, explique-nous.Since you know it, explain to us.
Évidence partagéeShared evidence.	Nous partons puisque tout est prêt.We are leaving since everything is ready.
Justification logiqueLogical justification.	Puisque la porte est ouverte, entrez.Since the door is open, come in.

The order of the clauses depends mainly on the logical value of the cause. Because and since often follow the main clause, while as tends to be placed at the beginning of the sentence. These conjunctions do not impose a particular verb tense, but the tenses must remain coherent with the logical sequence of facts.

IdéeIdea.	ExempleExample.
Ordre variableVariable order.	Nous rentrons parce qu’il pleut.We are going home because it is raining.
Comme en têteAs a fronted element.	Comme il faisait nuit, nous sommes partis.As it was dark, we left.
Cohérence logiqueLogical coherence.	Puisqu’il avait fini, il est sorti.Since he had finished, he went out.

The choice among these conjunctions depends on the level of language and the effect sought. Because is used mainly in informal or neutral speech, as is preferred in more formal writing, and as or since are often used to present a cause already established in the discourse. This stylistic flexibility is reflected in other word classes studied with Coordinating Conjunctions and Subordinating Conjunctions.

RégionRegion.	Mot ou expressionWord or expression.	Définition régionaleRegional definition.	ExempleExample.
Français courantEveryday French.	parce queBecause.	Le tour le plus naturel pour exprimer une cause directe dans la conversation.The most natural way to express a direct cause in conversation.	Il reste, parce que il a faim.There remains, because he is hungry.
Français écritWritten French.	carbecause.	Le tour le plus fréquent dans une justification formelle ou soignée.The most common phrasing in a formal or carefully polished justification.	Il reste, car le travail n’est pas terminé.There remains, because the work is not finished.
Français argumentatifArgumentative French.	puisquesince.	Le tour qui présente une cause tenue pour acquise ou évidente.The construction that presents a cause taken as given or obvious.	Puisque tu le sais, réponds.Since you know it, answer.

Causes must remain readable and must not accumulate unnecessarily in a single sentence. It is better to choose a single well-placed causal conjunction than to multiply the justifications, in order to preserve clarity and conciseness. The thus controlled causal links directly prepare the analysis of logical relations in Propositions and Subordinate Clauses.