You would use the subjunctive for hypothetical statements such as Si no lo hubieras dicho, habría sido mejor. Si no fueras a la escuela, nunca lo aprenderías. Si tuvieras más amigos, no serías tan solitaria. You use the past subjunctive for the verbs in the first clause, and the conditional for the second clause. If you are talking about a thing that could happen in the future, you would use the present indicative for the first clause, and present indicative, future, or imperative (command) for the second. Si no comes la cena, no puedes tener el postre. Si vienen a la fiesta, estará súper divertido. Si hay platos sucios, lávalos. Hope that helps!

Si [...], entonces [...]

Hey, Amber. There are four degrees of this structure, usually called "potencial". Degree 0. If you come, I give you the book (first verb in present and second verb in present). Degree 1. If you come, I will give you the book (first verb in present and second verb in future). Degree 2. If you came, I would give you the book (first verb in subjunctive and second verb in conditional) Though it is said that English does not have subjuntive, that is actually a mistake, it does have subjunctive, it just happens to be the past for most verbs, with the exception of "to be" where the subjunctive is really noticiable... If I were you...). Degree 3. If you had come, I would have given you the book (first verb under a subjuntive perfect tense and second verb under conditional perfect tense). These in Spanish are: Si tú vienes, yo te doy el libro. Si tú vienes, yo te daré el libro. Si tú vienieras, yo te daría el libro. Si tú hubieras venido, yo te habría dado el libro. I hope this is helpful.

You use the past subjunctive (not subjective) with "if clauses". The verb in the following clause would, more than likely, be followed by the conditional. Ejemplos: Si Miguel pudiera completarlo correctamente, yo no tendría que estar allí. - If Miguel were able to complete it correctly, I wouldn't have had to be there. If the sentence is not in the past tense, then do not use the subjunctive. Si Miguel lo puede completarlo correctamente, no tendré que estar allí. If MIguel can complete it correctly, I won't have to be there. The use of "then" doesn't work very well in either of these sentence in conjunction with the main clause.

Hola Amber; supongo que te refieres a reglas lógicas o de programación. Decimos "Si... entonces..." en el sentido de."si A... entonces B".