Si: condition et interrogation indirecte

The two-letter word si does two completely separate jobs in French. The first is what every learner meets first: it introduces a hypothetical condition — if in English. The second is less obvious from the outside: it introduces an embedded yes/no question — whether in English. The two uses share the same form but obey different grammar, especially with respect to which tenses are allowed after si.

There is also a third, rarer use as an intensifying adverb meaning so (si beau — so beautiful) and a fourth as a contradicting yes (— Tu ne viens pas ? — Si !), neither of which is a conjunction. This page covers all four uses, in order of frequency, and ends with the two iron rules that English speakers most often violate: no future after conditional si, and the que-replacement when chaining conditions.

Use 1: conditional si — if

This is the headline job. Si introduces the protasis (the if-clause) of a conditional sentence. The main clause (the apodosis) follows or precedes it. French has three canonical conditional patterns, distinguished by how realistic the hypothesis is.

Type 1 — real condition (likely / possible)

si + présent → présent, futur, or impératif in the apodosis.

This is the everyday conditional. The condition is real, the outcome is real (or close to real). English uses if + present + will / present / imperative, and French does exactly the same — except for one detail covered below.

Type 2 — unreal present condition (counterfactual now / unlikely future)

si + imparfait → conditionnel présent in the apodosis.

The English-speaker pitfall is calling the imparfait a past tense and being surprised that it appears in a sentence about the present. In conditional clauses, the imparfait is not signaling past time — it is signaling distance from reality. This is the same modal use of past forms that English shows in if I were (subjunctive past), and the parallel is exact.

Type 3 — unreal past condition (counterfactual past)

si + plus-que-parfait → conditionnel passé in the apodosis.

Note that the apodosis of a Type-3 sentence can drift into the conditionnel présent when the consequence applies to the present moment (as in the third example). French is less rigid than English on this point — mixing tenses across the two clauses is fine when the time references mismatch.

THE iron rule: never si + futur or conditionnel

After conditional si, the verb is in the présent, imparfait, or plus-que-parfait — never in the future, future perfect, conditional, or conditional perfect. This is the single most common transfer error from English (if you will come) and from learners overgeneralizing French tense logic.

The mnemonic taught in French schools is les si n'aiment pas les -rais (si doesn't like -rais endings — the conditional ending) and les si n'aiment pas les -ra (nor the future ending). It's a memorable rule, and it's absolute in the conditional use of si. (We'll see immediately below that the rule does not apply to the question use of si.)

Use 2: indirect yes/no question — whether

When you embed a yes/no question after a verb of asking, wondering, knowing, or telling, French uses si — equivalent to English whether (or unstressed if).

Look at that last example. Tu seras is the futur simple — and it is fully correct after si here, because this si is the question conjunction, not the conditional. The future-prohibition rule applies only to the conditional si, not the interrogative si. This is the crucial difference between the two roles.

The test: replace si with whether in your English translation. If it works, you have the question si and any tense (including future and conditional) is allowed. If only if works (in the if-then hypothetical sense), you have the conditional si and the future is forbidden.

The interrogative si also accepts the conditional in reported speech:

That conditionnel is correct because reported speech uses tense backshift, and si here is interrogative.

si oui ou non — whether or not

The fixed phrase si oui ou non makes the dichotomy explicit, like English whether or not.

This phrase is moderately formal. In casual speech, French speakers often just use a plain si: je ne sais pas s'il va venir.

Use 3: replacement of a second si in a chain — que + subjunctive

When two conditions are joined by et or ou, the second cannot be introduced by another si. French replaces it with que + subjunctive.

This is the rule that overlaps with the que page (Role 7). It is mentioned here because, from the si side, the rule is: never repeat si in a coordinated chain — switch to que + subjunctive after the connector. Stylistically, you may also restart the sentence: Si je viens, et si je te vois… with a clear pause and comma is acceptable in writing but feels heavier than the que-form.

Use 4: intensifying adverb — so

This si is not a conjunction. It is an adverb that intensifies an adjective or another adverb, equivalent to English so.

This si tends to feel slightly literary or careful. In casual speech, tellement is more common (elle est tellement belle). The two are interchangeable in meaning when used as adverbs of degree, but si is shorter and reads as more refined.

When si (so) is followed by que, it forms a result clause — si X que Y (so X that Y).

The si … que result construction always takes the indicative in the result clause. (Compare with si X que Y in concessive use — however X you may be — which takes the subjunctive and is rarer.)

Use 5: contradicting yes — si (not oui)

Strictly speaking this is not the conjunction si, but English speakers meet it constantly and confuse it with the conditional. When someone makes a negative statement or question and you want to affirm the positive, you answer si, not oui.

This si signals that you are correcting a negative presupposition. Oui would be wrong here — oui answers questions framed positively. There's no English equivalent — English uses the same yes whether the question was positive or negative, and listeners disambiguate from context. French has the dedicated si for the contradiction case, and using oui in a contradiction sounds odd to native ears.

si under elision

si elides to s' only before il and ils. Before any other word — including elle, elles, on, un — it stays as si.

This is a quirk: most other words ending in -i never elide (qui, mi-, ni). Si elides only before il/ils. Before elle, you write the full si even though the spoken result is /sɛl/.

Common Mistakes

Key Takeaways

The two main jobs of si — condition and embedded question — share a form but follow different rules. After conditional si, the verb is in the présent, imparfait, or plus-que-parfait, and the future or conditional is absolutely forbidden in the same clause. After interrogative si (meaning whether), all tenses are allowed, including the future and the conditional. Add the rule that a second si in a chain becomes que + subjunctive, the contradicting si answer, and the intensifier si meaning so, and you have the full picture of what this small word does. Mastering the no-future-after-conditional-si rule is the single biggest fluency upgrade for English-speaking learners at this level.