Although the rules of grammar for conditional sentences — usually those using the word si ("if") — can get fairly complex, in the vast majority of cases the decision of which verb tense to use after si is easy to remember.
The first thing is to remember that except in very rare cases, si is never followed by a verb in the present-tense subjunctive mood.
That said, there are basically two types of si clauses that become part of a sentence:
- Sentences in which the condition is likely or reasonably likely. This is known grammatically as an open condition. For example, in the clause si llueve ("if it rains"), rain is seen as a distinct possibility.
- Sentences in which the condition is contrary to fact or is unlikely. For example, the clause si lloviera can be translated as "if it were to rain." Note the difference in meaning from the example above; in this case, while rain is a possibility, it is seen as unlikely. An example of a contrary-to-fact condition is a clause such as si yo fuera rico, "if I were rich." Grammatically, contrary-to-fact and unlikely conditions are treated the same way.
The correct verb tense following si can be observed in the above examples. In open conditions, conditions where the possibility is reasonably likely, si is followed by the present indicative tense (the most common tense, probably the first one you learned as a Spanish student). If the condition is unlikely or false, a past subjunctive (usually the imperfect subjunctive) is used. This is the case even when the condition is something that refers to the present.
Here are some examples of open conditions:
- Si tengo dinero, me iré de viaje. If I have money, I'll go on a trip.
- Si la casa es usada, le aconsejamos que un profesional la inspeccione. (If the house is used, we advise that you have a professional inspect it.)
- Si sales, salgo también. (If you leave, I'm leaving too.)
- Si gana Sam, voy a llorar. (If Sam wins, I'll cry.)
Here are some examples of unlikely or contrary-to-fact conditions:
- Si yo fuera tú, tomaría una responsabilidad propia. (If I were you, I would take appropriate responsibility.)
- Si yo tuviera dinero, iría al cine. (If I had the money, I would go to the movies.)
- Si ella hubiera tenido dinero, habría ido al cine. (If she had had the money, she would have gone to the movies.)
- Si ganara Sam, lloraría. (If Sam were to win, I'd cry.)
In Spanish, as in English, the si clause can either precede or follow the rest of the sentence. Thus a sentence such as si llueve voy de compras ("if it rains I'm going shopping") is the equivalent of voy de compras si llueve ("I'm going shopping if it rains").