What are the three types of si clause in Spanish?

Real or open conditions use si plus the present indicative, answered by the present, the future or an imperative: si llueve, me quedo en casa (if it rains, I stay home), si tienes tiempo, llámame (if you have time, call me). Unreal present or future conditions use si plus the imperfect subjunctive, answered by the simple conditional: si tuviera dinero, viajaría por el mundo (if I had money, I would travel the world). Unreal past conditions - counterfactuals about what did not happen - use si plus the pluperfect subjunctive, answered by the conditional perfect: si hubiera estudiado, habría aprobado (if I had studied, I would have passed). The type you pick tracks how real the if is: a live possibility, an imagined present, or an undone past.

Why can you never say 'si tendría' or 'si tenga'?

Because Spanish bans both the simple conditional and the present subjunctive directly after si. The if half of an unreal sentence takes the imperfect subjunctive, and the conditional lives in the other half: not si tendría dinero but si tuviera dinero, and the tendría goes with the result - tendría vacaciones si pudiera. The present subjunctive never follows si at all in a plain conditional; si tenga is simply not Spanish. This is the single most common mistake English and French speakers make, because their languages let a conditional-looking form sit in the if clause. In Spanish it cannot. If you have written si followed by a -ría form or a present subjunctive, it is wrong.

What does 'como si' take, and why?

Como si (as if) always takes the subjunctive, and only the past ones - the imperfect subjunctive for a present comparison, the pluperfect subjunctive for a past one. Habla como si fuera el jefe (he talks as if he were the boss), me miró como si hubiera visto un fantasma (she looked at me as if she had seen a ghost). The reason is that como si sets up an unreal comparison by definition - he is not the boss, she did not see a ghost - so it behaves like the unreal half of a conditional and never takes the indicative. There is no como si es or como si fue; it is always como si fuera or como si hubiera.

Si Clauses in Spanish: Real and Unreal Conditions

A si clause is an if-then sentence, and Spanish is unusually neat about them. Every conditional falls into one of three boxes, decided by how real the if is: a genuine live possibility, a hypothesis you are only imagining, or a counterfactual about a past that did not happen. Each box has a fixed pairing of tenses, and once you know which box you are in, the grammar is mechanical.

This page assumes you can already build the pieces. If tuviera, viajaría, hubiera estudiado and habría aprobado look unfamiliar, go and meet the imperfect subjunctive and the conditional first, then come back.

The golden rule, first, because it outranks everything

Before the three types, the one thing you must never do. Nothing that looks like a conditional and nothing that looks like a present subjunctive ever follows si directly.

Never si tendría - the simple conditional is banned after si.
Never si tenga - the present subjunctive is banned after si.

This is the single most common mistake, and it is worth being blunt about: if you have written si followed by a -ría form (tendría, iría, sería) or a present subjunctive (tenga, vaya, sea), it is wrong, full stop. The conditional lives in the other half of the sentence. The si half of an unreal condition takes the imperfect subjunctive. English lets you say "if I would have", French lets a conditional shape sit in the if clause, and both instincts wreck Spanish. Kill them now and the three types below fall into place.

Type 1: real, open conditions (it might genuinely happen)

When the if is a live possibility - it really could rain, you really might have time - use si + present indicative. The result half takes the present, the future, or an imperative.

Si llueve, me quedo en casa. (If it rains, I stay home.) - present result.
Si llueve, me quedaré en casa. (If it rains, I will stay home.) - future result.
Si tienes tiempo, llámame. (If you have time, call me.) - imperative result.
Si estudias, apruebas. (If you study, you pass.) - a general truth.

Note what does not happen here: even though English can say "if it will rain", Spanish never puts the future after si. The if half stays in the present indicative - si llueve, not si lloverá - and the future, if you want it, goes in the result half. This is the everyday conditional, the one for real plans and real possibilities.

Type 2: unreal present or future (a hypothesis you are imagining)

When the if is unreal - you do not actually have the money, you are only supposing - drop into si + imperfect subjunctive, and answer with the simple conditional.

Si tuviera dinero, viajaría por el mundo. (If I had money, I would travel the world.)
Si fuera rico, compraría una casa en la playa. (If I were rich, I would buy a house on the beach.)
Si pudiera, te ayudaría. (If I could, I would help you.)
¿Qué harías si ganaras la lotería? (What would you do if you won the lottery?)

The pattern is fixed: imperfect subjunctive in the if half, conditional in the result half. The money, the wealth, the lottery win are all imagined, not real, and that is exactly what the imperfect subjunctive marks. This is where learners try to write si tendría dinero by copying the English "if I would have"; resist it. It is si tuviera, with the viajaría waiting in the other half.

Type 3: unreal past (a counterfactual - it did not happen)

When the if is about a past that did not happen - you did not study, so you did not pass - use si + pluperfect subjunctive, answered by the conditional perfect.

Si hubiera estudiado, habría aprobado. (If I had studied, I would have passed.)
Si hubieras venido, te habrías divertido. (If you had come, you would have had fun.)
Si lo hubiera sabido, no habría dicho nada. (If I had known, I would not have said anything.)
Si no hubiera llovido, habríamos ido a la playa. (If it had not rained, we would have gone to the beach.)

Both halves are compound: hubiera + past participle in the if, habría + past participle in the result. The whole sentence looks back at something that is now impossible to change - the studying that never happened, the visit that was never made. This is the regret tense, and it is the one where English speakers pile up "if I would have had", which maps onto nothing in Spanish. It is si hubiera estudiado, cleanly.

The three types on one card

Type	If half (si...)	Result half	Example
Real / open	present indicative	present / future / imperative	Si llueve, me quedo en casa.
Unreal present	imperfect subjunctive	simple conditional	Si tuviera dinero, viajaría por el mundo.
Unreal past	pluperfect subjunctive	conditional perfect	Si hubiera estudiado, habría aprobado.

Read down the middle column and the rule enforces itself: after si you only ever see the present indicative, the imperfect subjunctive, or the pluperfect subjunctive. Never a conditional, never a present subjunctive.

The -ra and -se swap

The imperfect subjunctive has two sets of endings, -ra and -se, and in a si clause they are interchangeable. Both are correct, both mean the same thing.

Si tuviera dinero, viajaría. = Si tuviese dinero, viajaría.
Si fuera rico... = Si fuese rico...
Si hubiera estudiado... = Si hubiese estudiado...

The -ra forms are more common in speech across most of the Spanish-speaking world; the -se forms lean formal and literary, and are still lively in Spain. Pick one and be consistent within a sentence, but know that when a text throws tuviese or hubiese at you, it is doing exactly what tuviera and hubiera do. Note the conditional in the result half does not have this choice - it is always -ría, never a -se form.

The halves can swap, and the comma follows

The si half does not have to come first. You can lead with the result instead, and the meaning is identical.

Si llueve, me quedo en casa. = Me quedo en casa si llueve.
Si tuviera dinero, viajaría. = Viajaría si tuviera dinero.
Si hubiera estudiado, habría aprobado. = Habría aprobado si hubiera estudiado.

The rule for the comma is simple: when the si clause comes first, put a comma between the halves; when the result comes first, no comma. So si llueve, me quedo but me quedo si llueve. It mirrors the English "if it rains, I stay" versus "I stay if it rains", and the logic is the same.

como si always takes the subjunctive

One close cousin worth pinning down. Como si (as if) sets up an unreal comparison by its very meaning, so it always takes the past subjunctive - the imperfect subjunctive for a present comparison, the pluperfect subjunctive for a past one. It never takes the indicative.

Habla como si fuera el jefe. (He talks as if he were the boss.) - he is not the boss.
Me trata como si no me conociera. (She treats me as if she did not know me.)
Me miró como si hubiera visto un fantasma. (She looked at me as if she had seen a ghost.) - a past comparison.

There is no como si es and no como si fue. Because "as if" always introduces something contrary to fact, it behaves like the unreal half of a conditional: como si fuera, como si hubiera, and nothing else.

Worked examples

Si vienes a la fiesta, te presento a mis amigos. (If you come to the party, I will introduce you to my friends.) - real, present result.
Si tienes hambre, come algo. (If you are hungry, eat something.) - real, imperative.
Si fuera tú, no lo haría. (If I were you, I would not do it.) - unreal present.
¿Qué harías si te tocara la lotería? (What would you do if you won the lottery?) - unreal present, -ra form.
Si hubieras llegado antes, habrías visto el final. (If you had arrived earlier, you would have seen the ending.) - unreal past.
Si no hubiese perdido el tren, no habría llegado tarde. (If I had not missed the train, I would not have arrived late.) - unreal past, -se form.
Gasta como si fuera millonario. (He spends as if he were a millionaire.) - como si, imperfect subjunctive.

Common mistakes English speakers make

Putting the conditional after si. The big one. Writing si tendría dinero or si iría copies the English "if I would have". Spanish forbids it: the si half of an unreal condition takes the imperfect subjunctive (si tuviera, si iría becomes si fuera), and the conditional stays in the result half. If si is followed by a -ría form, rewrite it.

Putting the present subjunctive after si. The other banned form. Si tenga tiempo and si venga are not Spanish; a plain conditional never takes the present subjunctive after si. Type 1 wants the present indicative - si tengo tiempo, si vienes - so it is the ordinary present, not the subjunctive.

Using the future after si. English says "if it will rain"; Spanish never does. The if half of a real condition is present indicative - si llueve, not si lloverá - and the future lives only in the result half: si llueve, me quedaré.

Mixing the pairings. Each type has a fixed partner. Si tuviera dinero, viajo mismatches an unreal if with a real result; it should be viajaría. Likewise si hubiera estudiado, aprobaría blends past and present unreal - for a past counterfactual it is habría aprobado. Match the two halves: imperfect subjunctive with simple conditional, pluperfect subjunctive with conditional perfect.

Get the golden rule in place - no conditional and no present subjunctive after si - and the three types are just three fixed pairs. Match the box to how real the if is, mind the comma when the si comes first, and remember that como si is always subjunctive. That is the whole system.