1. HYPOTHETICAL SITUATIONS IN THE FUTURE OR THE PRESENT THAT ARE CONTRARY TO FACT OR VERY UNLIKELY.
   1. Si + imperfect subjunctive+ the conditional
      1. Si lloviera (if it were to rain) no saldría. (I wouldn't go out,)
      2. Yo le daría la respuesta, si la supiera. I would give you the answer, if I knew it.
2. HYPOTHETICAL SITUATIONS IN THE PAST THAT ARE IMPOSSIBLE OR CONTRARY TO FACT.
   1. Si + pluperfect subjunctive+ the conditional perfect or pluperfect subjunctive
      1. Si hubiera llovido no habría salido. (or no hubiera salido.) If it had rained (it didn't) I wouldn't have gone out.
3. COMO SI - AS IF
   1. NB: Como si is always followed by a past time subjunctive. Imperfect subjunctive for present time hypothesis. Past perfect subjunctive for past time hypothesis.
      1. Juan hablaba como si nadie lo hubiera visto. John was talking (talked) as if no one had seen him.
      2. Los adolescentes hablan como si tuvieran todas las respuestas. Teenagers talk as if they have all the answers.
4. NB: 'Si' (if) is very rarely followed by the present subjunctive BUT rules the rule can be broken.
   1. Especially with negative 'si' clauses (such as - no sé si ... ) the present subjunctive is sometimes used to express more doubt, 'si' then takes on the meaning of 'whether' more than 'if' .
      1. No sé si él esté en casa. I don't know whether he'll be at home.
      2. No sé si él está en casa o no. I don't know if he's at home or not.