When you embed a clause inside another sentence — I think (that) she's right, I asked whether they were coming, I'm not sure if it matters — English uses three small words almost interchangeably: that, if, whether. English even lets you drop that entirely: I think she's right. Spanish allows none of this looseness. It uses exactly two complementisers, que for statements and si for yes/no questions, and neither is ever optional. Choosing one over the other, and never dropping either, is the foundation of all complex sentence-building in Spanish.
This page covers the two introducers in detail: what they signal, when each appears, what mood they select, and the specific traps that catch English speakers.
Que — the declarative complementiser
Que introduces an embedded statement after verbs of saying, thinking, believing, knowing, wanting, fearing, hoping, denying — almost any verb that takes a finite clause as its object or complement.
Sé que viene mañana en el AVE de las nueve.
I know he's coming tomorrow on the nine o'clock high-speed train.
Creo que es buena idea preguntarle al vecino.
I think it's a good idea to ask the neighbour.
Le dijo a su madre que no le esperara para cenar.
He told his mother not to wait for him for dinner.
Quiero que me escuches con atención cinco minutos.
I want you to listen to me carefully for five minutes.
Notice that que appears in all four sentences, regardless of whether the embedded verb is in the indicative (first two) or the subjunctive (second two). The mood is decided by the meaning of the main verb — saber and creer assert, decir (in this command sense) and querer impose — but the introducer que is constant.
Que is obligatory — never drop it
This is the single most important rule on this page, and it is the rule English speakers break most often. English freely omits that:
I think she's right. He said he was tired. I know you can do it.
In each case the that is optional in English. In Spanish the equivalent que is not optional, and dropping it produces an ungrammatical sentence that no native speaker would say.
❌ Creo es buena idea.
Ungrammatical — no native speaker says this. English-speaker calque.
✅ Creo que es buena idea.
I think it's a good idea.
❌ Dijo estaba cansado.
Ungrammatical — the que is mandatory.
✅ Dijo que estaba cansado.
He said he was tired.
Train your ear to hear the absence of que as a missing tooth. Until omitting it feels physically wrong, you are still translating from English.
Que keeps its place after prepositions and after verbs of command
Estoy seguro de que vendrá si se lo pido.
I'm sure he'll come if I ask him.
Le pidió a su hijo que recogiera la habitación antes de salir.
She asked her son to tidy up his room before going out.
After a verb + preposition like estar seguro de, insistir en, alegrarse de, the preposition and que both appear, side by side: de que, en que, a que. Dropping the preposition (❌estoy seguro que vendrá) is queísmo; doubling it by adding de where none is needed (❌creo de que vendrá) is dequeísmo. Both are recognised non-standard errors in peninsular Spanish.
Si — the embedded yes/no question
When the embedded content is a yes/no question — "I asked whether…", "I don't know if…", "I wonder whether…" — the introducer is si. The embedded verb is always in the indicative.
Me preguntó si tenía hambre, y la verdad es que sí.
She asked me if I was hungry, and the truth is I was.
No sé si vendrá esta noche, no me ha confirmado nada.
I don't know if he'll come tonight, he hasn't confirmed anything.
Quería saber si quedaban habitaciones libres para el fin de semana.
She wanted to know whether there were any rooms free for the weekend.
Dudo si llamarlo ahora o esperar hasta mañana por la mañana.
I'm hesitating whether to call him now or wait until tomorrow morning.
Verbs that commonly take embedded si in peninsular Spanish: preguntar, no saber, saber (negated or interrogative), querer saber, dudar, ignorar, preguntarse ("wonder"), averiguar, comprobar.
Si always takes the indicative
This is the second big rule, and the one that catches learners who have learned (correctly) that si + verb often takes the subjunctive in conditional sentences. Different si, different rule. The embedded-question si refuses the subjunctive.
❌ No sé si venga mañana.
Ungrammatical — embedded yes/no questions take the indicative, even with future reference.
✅ No sé si vendrá mañana.
I don't know if he'll come tomorrow.
The mistake is so common it has a name in Spanish-teaching circles: subjuntivo intruso ("intrusive subjunctive"). It crops up whenever the embedded content is uncertain or future, because learners associate uncertainty with the subjunctive. But the mood is decided by the structural function (embedded question), not the semantic feel.
Two faces of si: a head-to-head comparison
The same form si appears in two completely different constructions. Distinguishing them is non-negotiable for B1+ writers.
| Construction | Embedded si (whether) | Conditional si (if) |
|---|---|---|
| Function | Introduces an indirect yes/no question, complement of a verb of asking/knowing | Introduces the protasis of a conditional sentence |
| Translation | "whether" or "if (whether)" | "if (in case)" |
| Mood with present/future reference | Indicative (present, future, conditional) | Indicative (present) — never future |
| Mood with hypothetical reference | Conditional indicative | Imperfect subjunctive |
| Example | No sé si vendrá. | Si viene, le digo. |
| Counterfactual example | No sé si vendría aunque le invitara. | Si viniera, le diría. |
The decisive test: try replacing si with whether. If the sentence still makes sense, it's the embedded-question si and the verb goes in the indicative. If you can only say if (in the sense of in case, provided that), it's the conditional si and the verb may go in the imperfect subjunctive when hypothetical.
No sé si llamarte ahora o luego.
I don't know whether to call you now or later. (embedded question)
Si te llamo ahora, ¿lo coges?
If I call you now, will you pick up? (conditional)
Embedded wh-questions: qué, dónde, cuándo, cómo…
When the embedded question is a wh-question rather than a yes/no question, the wh-word itself is the introducer — and it keeps its written accent to mark its interrogative function.
Me preguntó qué quería para comer.
He asked me what I wanted for lunch.
No me dijo cuándo iba a llegar exactamente.
He didn't tell me exactly when he was going to arrive.
Pregúntale dónde ha dejado las llaves.
Ask him where he's left the keys.
The accent is critical: qué, dónde, cuándo, cómo, quién, cuál (with accent) are interrogatives; que, donde, cuando, como, quien, cual (without accent) are relatives. Embedded questions take the accented form.
❌ Me preguntó que quería para comer.
Wrong — without an accent, 'que' is a declarative complementiser, but the embedded clause here is a wh-question. The interrogative qué needs its accent.
✅ Me preguntó qué quería para comer.
She asked me what I wanted for lunch.
Embedded wh-questions can take either the indicative (the most common choice) or, more rarely, the subjunctive when the question itself is deliberative or rhetorical. The full picture is on the indirect questions and mood page.
The colloquial que si doubling
A peninsular conversational feature: in indirect speech, the que of declarative reporting and the si of embedded questions can stack into a single que si, used as a kind of conversational throat-clear when relaying what someone said.
Me llamó mi madre y me dijo que si podía pasarme luego por su casa.
My mum called and asked if I could drop by her place later. (informal/colloquial)
Va y me dice que si le presto el coche el fin de semana, ¡como si tuviera coche!
And then he goes and asks if I'll lend him the car for the weekend — as if I had a car! (colloquial peninsular)
This que si construction is a marker of casual register. It is fine in conversation and in dialogue in fiction. It would not appear in journalism or formal writing, which would use either me preguntó si… or me dijo que….
Mood after embedded si with future reference
The behaviour of embedded si with future reference is the single most common pitfall on this page, and it deserves its own treatment.
In peninsular Spanish, the embedded si takes the future indicative when reporting uncertainty about a future event:
Me pregunto si vendrá Marta a la cena del sábado.
I wonder whether Marta will come to dinner on Saturday.
Aún no sabemos si llegaremos a tiempo para coger el último tren.
We still don't know if we'll be in time to catch the last train.
Compare with the conditional si, which uses the present indicative for the same time frame:
Si viene Marta a la cena, le hago una paella.
If Marta comes to dinner, I'll make her a paella. (conditional — NOT *si vendrá)
Same target tomorrow night, two different si-constructions, two different verb forms. The grammar selects them; the time reference does not.
Comparison table: que vs si at a glance
| Feature | Que (declarative) | Si (embedded yes/no question) |
|---|---|---|
| Translates as | "that" (often omitted in English) | "whether" / "if" |
| Used after | verbs of saying, thinking, knowing, wanting, fearing… | verbs of asking, wondering, not knowing |
| Mood inside the clause | indicative or subjunctive (decided by the main verb) | always indicative |
| Optional? | No — never dropped | No — never dropped |
| Carries an accent? | No — que is unstressed | No — si is unstressed (the accented sí means "yes") |
A final caution about accent marks: the conjunction si (= "whether/if") is never written with an accent. Adding one — ❌no sé sí vendrá — turns it into the adverb sí meaning "yes," producing a different sentence (No sé sí, vendrá would mean something like "I don't know 'yes', he's coming"). The accent on interrogative wh-words is obligatory; the accent on the conjunction si would be wrong.
Common Mistakes
❌ Creo es buena idea.
Wrong — Spanish never drops 'que' the way English drops 'that'.
✅ Creo que es buena idea.
I think it's a good idea.
❌ Me preguntó que tenía hambre.
Wrong — embedded yes/no questions take 'si', not 'que'. (Without accent, 'que' would also be wrong here.)
✅ Me preguntó si tenía hambre.
She asked me if I was hungry.
❌ No sé si venga mañana.
Wrong — the embedded-question 'si' always takes the indicative, never the subjunctive. Future reference takes the future indicative.
✅ No sé si vendrá mañana.
I don't know if he'll come tomorrow.
❌ Me preguntó que quería para comer.
Wrong — embedded wh-questions need the accented interrogative 'qué'. Without the accent, 'que' would be the declarative complementiser, which doesn't fit a wh-question.
✅ Me preguntó qué quería para comer.
She asked me what I wanted for lunch.
❌ Estoy seguro que vendrá.
Wrong (queísmo) — 'estar seguro' selects the preposition 'de' before 'que'.
✅ Estoy seguro de que vendrá.
I'm sure he'll come.
Key takeaways
- Spanish has two complementisers: que for declaratives and si for embedded yes/no questions. Wh-questions keep their accented wh-word as the introducer.
- Neither que nor si is ever dropped, no matter how natural omitting that feels in English.
- The mood after que is decided by the main verb (assertion → indicative, wish/doubt/evaluation → subjunctive). The mood after embedded si is always indicative.
- The same form si belongs to two completely different constructions — the embedded-question si and the conditional si — that should never be confused. The whether-test distinguishes them.
- Future reference in an embedded si uses the future indicative (si vendrá), not the present subjunctive (❌si venga).
- Embedded wh-words keep their accent (qué, dónde, cuándo, cómo…) — the accent is what flags them as interrogatives.
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- Cláusulas sustantivas: completivasB1 — A noun clause does the work of a noun — subject, object, or complement of a preposition — and the mood inside it (indicative vs subjunctive) is decided by what the main verb asserts about reality.
- Cláusulas subordinadas: visión generalB1 — The master taxonomy of Spanish subordination: substantive clauses (objects: 'que viene'), adjective clauses (relatives: 'que viene'), and adverbial clauses (temporal, causal, conditional, concessive, purpose). How each is introduced and the mood-selection rules that govern them.
- Dequeísmo: 'pienso de que' (error)B2 — The classic Spanish hypercorrection pair: dequeísmo (inserting 'de' where no preposition is licensed — *pienso de que es bueno) and queísmo (omitting 'de' where it is required — *me alegro que vengas). How to diagnose and avoid both with the 'eso' substitution test.
- Estilo indirecto: visión generalB1 — Reported speech in Spanish reshapes a quote along three dimensions — tenses, pronouns, and time-place adverbials — and the reporting verb decides what introduces the clause: que, si, a wh-word, or que + subjunctive.
- Modo en interrogativas indirectasB2 — Mood choice in embedded questions — when 'preguntar', 'saber', 'no saber', and 'querer saber' allow or require subjunctive, and how Peninsular Spanish handles the indicative–subjunctive boundary.
- Oraciones condicionales: guía completaB1 — A full reference for Spanish conditional sentences — the four classical types plus mixed conditionals, organised by how real the speaker considers the condition: factual, real-future, hypothetical, or counterfactual.