1. “Si” Clauses ============ Pero/sino/sino que

2. Imperfect Subjunctive
Start with the third-person plural form of the preterit.
Drop the -ron ending to establish the verb’s imperfect subjunctive base.
Imperfect Subjunctive Verb Endings Yo
-ra Tú
-ras
él/ella/ello/uno
Usted
nosotros/nosotras
-ramos
vosotros/vosotras
-rais
ellos/ellas
-ran
Ustedes

3. Pluperfect Subjuctive
The pluperfect subjunctive is a compound verb formed with the imperfect subjunctive of the auxiliary verb haber + the past participle
-RA conjugation of TENER yo
hubiera tenido   
nosotros
hubiéramos tenido tú
hubieras tenido
vosotros
hubierais tenido él
ellos
hubieran tenido

4. Real vs. unreal conditions
A real condition is one which may actually come about or at least is viewed as a possibility; thus, in Spanish, the indicative is
Si Marcos está su cuarto, está escuchando música. If he is in his room [he may actually be in his room], he is listening to music.
Si nieva mucho, podré esquiar. If it snows a lot [it may really snow], I can ski.

5. Real vs. unreal conditions
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 uses a form of the past subjunctive
AND
the main uses a form of the conditional tense.

6. Real vs. unreal conditions
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 would buy a car.
¿Qué harías si fueras presidente?
What would you do if you were president?
Si Juana estuviera aquí, ¿le dirías la verdad?
If Juana were here, would you tell her the truth?

7. Real vs. unreal conditions
Si la hubiera visto, habría dicho algo. If I had seen her I would have said something.
Si hubieras venido, te habrías divertido mucho. If you had 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?
Past time situations:
Past perfect subjunctive in the “if” clause,
and
The conditional perfect in the main clause

8. IF the book is expensive, Result Clause
“If” Clause
present (indicative)
Si el libro es caro,
IF the book is expensive,
Result Clause
present (indicative)
Yo no lo compro
I don’t buy it
Future (indicative)
Yo no lo compraré
I won’t buy it

9. imperfect (subjunctive)
“If” Clause
imperfect (subjunctive)
Si tuviera dinero,
IF I had money,
Result Clause
Conditional (indicative)
Yo lo compraría
I would buy it

10. Si el libro hubiera sido caro, Result Clause
“If” Clause
pluperfect
(subjunctive)
Si el libro hubiera sido caro,
IF the book had been expensive expensive,
Result Clause
conditional perfect (indicative)
Yo no lo habría comprado
I wouldn't have bought it

11. Como si… The: Or is also used after como si, meaning as if…
imperfect subjunctive Or
the pluperfect subjunctive
is also used after como
si, meaning as if…
Le preguntaron a José como si supiera todas las respuestas (No las sabe todas) They asked José as if he knew all the answers (He doesn’t know all of them)
Me habló como si yo hubiera hecho algo inapropriado. (No lo hice) He spoke to me as if I had done something inappropriate. (I didn’t)

12. Pero/sino/sino que They all mean “BUT” in the following contexts:
1. Use PERO (but) when the second part of the sentence does not correct the first. Also most time the first part is affirmative.
“Me gustan las peliculas pero no quiero ir”
“El profesor no es bueno pero no da examenes”
2. Use SINO (but rather) when the first part of the sentence is negative and is followed by a noun, adjective or adverb that corrects the same in the first part
“No quiero ir a cine sino a casa” “No soy alto sino bajo”
3. Use SINO QUE (but instead) if the first part of the sentence is negative and it is followed by a new verb.
“ No quiero ir al cine sino que prefiero ir a casa”
“ Jose no apoya al nuevo candidato sino que vota por el ultimo”