Si + subjunctive... When is it appropriate ?
I had thought (and indeed been told on SD) that in Spanish when you form a clause beginning with 'si', your shouldn't use the subjunctive, and this has been borne out by most of what I've read or heard.
However, I've encountered the following phrase prefaced with 'si', which does take the subjunctive, and I wonder why it's different:
- Me habría ido mejor si no hubiera hecho tanto calor = It would've been better for me if it hadn't been so hot...
Muchisimas gracias por ayudarme 
6 Answers
It is only the present subjunctive that is not used after si, the past subjunctive is used all the time. See this link, in which I brought two relevant parts over. Please remember that when they talk about these unreal, contrary to fact, hypothetical situations that they do not necessarily have to be that hypothetical any more in Spanish than English, it’s just the spin you want to put on things, like the English:
If you were to give me that it would be nice. Were to give is past subjunctive- and clearly by framing it that way I do not think it impossible, instead I am just using it to more politely frame a request.
http://users.ipfw.edu/jehle/courses/condic.htm
I will tell you this guy has a number of very good posts on topics: http://users.ipfw.edu/jehle/S210.HTM
In contrast, an unreal or contrary-to-fact condition is one which will not come about or is viewed as being completely hypothetical. In this case, the “if” clause in normally in a past subjunctive tense, and the main verb is in a conditional tense.
Present or future time situations. The imperfect subjunctive is used in the “if” clause, and the conditional in the main clause:
Si yo fuera rico compraría un coche. If I were rich [I am not rich] I would buy a car. ¿Qué harías si fueras presidente? What would you do if you were president? [you aren't] Si Juana estuviera aquí, ¿le dirías la verdad? If Juana were here [she isn't here], would you tell her the truth?
Past time situations. Past perfect subjunctive in the “if” clause, conditional perfect in the main clause:
Si la hubiera visto, habría dicho algo. If I had seen her [I didn't see her] I would have said something. Si hubieras venido, te habrías divertido mucho. If you had come [you didn't come] you would have had a great time. ¿Habrías ido a la fiesta si yo la hubiera planeado? Would you have gone to the party if I had planned it? [I didn't plan it]
x
The present subjunctive is NOT used after si (“if”)!
Again, this last is for students, in Rac's link you can see where Lazarus says it is on rare occasion, but it is a good rule until you are so advanced you don't need rules.
Like in English:
If I were ....
Would this help?
Hello Falda ,
I believe that you need to use the imperfect subjunctive following the como si (as if ) construction, as illustrated in the example below:.
ex: Cuando le vi en la calle esta mañana, le saludé, pero me miró como si no me reconociera.
When I saw her in the street this morning I greeted her, but she looked at me as if she didn't recognise me!
The como si = as if /as though ,is referring to a hypothetical situation , as if something were true so takes the imperfect subjunctive.
The Oxford English /Spanish Dictionary confirms this, as it gives the entry under the Spanish word como: como si (+ subj) as if, as though
I hope this helps 
Corrijan mi español si es necesario, por favor
I hope this doesn't cause further confusion. I'm trying to learn too.
Your example is a hypothetical sentence= subjunctive.
Si no takes the subjunctive, even in the present, it is si, followed by a positive sentence that does not.
Sorry, this is not a very good explanation, I wonder where our Kiwi is, she was good at explaining grammar.
