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.
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.)
- Si yo fuera tú, tomaría una responsabilidad propia. (If I were you, I would take appropriate responsibility.)
- Si yo tuviera dinero, habría ido al cine. (If I had the money, I would go to the movies.)
- Si ella hubiera tenido dinero, iría 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.)