The Conditional, Conditional Perfect, and “If” Clauses

The conditional
Forms of the conditional. To form the conditional, use:
The same stem as for the future tense (normally, the infinitive; a list of irregular verbs is given below).
The same endings as for the imperfect of -er and -ir verbs: -ía, -ías, -ía, -íamos, -íais, -ían.
hablar

comer

vivir

hablaría hablaríamos comería comeríamos viviría viviríamos
hablarías hablaríais comerías comeríais vivirías viviríais
hablaría hablarían comería comerían viviría vivirían
Verbs with irregular future and conditional stems:

decir to say dir- diría, dirías, diría, ...
haber there to be [impersonal];
to have [helping verb] habr- habría, habrías, habría, ...
hacer to make, do har- haría, harías, haría, ...
poder to be able podr- podría, podrías, podría, ...
poner to put, place, set pondr- pondría, pondrías, pondría, ...
querer to want, love querr- querría, querrías, querría, ...
saber to know [a fact] sabr- sabría, sabrías, sabría, ...
salir to leave, go out saldr- saldría, saldrías, saldría, ...
tener to have tendr- tendría, tendrías, tendría, ...
valer to be worth valdr- valdría, valdrías, valdría, ...
venir to come vendr- vendría, vendrías, vendría, ...
Usage. There are several ways in which the conditional is used in Spanish:
To indicate future time within the past. (Remember that the conditional is a combination of future stem with imperfect endings.)
Dije ayer que lo haríamos hoy. Yesterday I said that we would [were going to] do it today.
The same idea could also be expressed with the imperfect of ir plus a plus the infinitive:

Dije ayer que lo íbamos a hacer hoy. Yesterday I said that we were going to do it today.
To indicate conjecture or probability in past time (roughly an equivalent of probablemente plus the imperfect).
¿Dónde estaría María anoche? Where do you think Mary was last night?
Estaría en casa. She was probably at home.
To indicate deference or softening of a statement or request. Compare these three examples:
Quiero cinco dólares. I want $5.00. [forceful, present tense]
Querría cinco dólares. I would like $5.00. [deferential, conditional]
Quisiera cinco dólares. I would like $5.00. [almost apologetic, imperfect subj.]
NOTE: The past subjunctive was used in the third example above to indicate deference, politeness, or an almost apologetic tone. This usage is found primarily with the verbs querer, deber, and poder:

¿Pudieras hacerlo? Could you (possibly) do it?
Debieras estudiar más. (Maybe) you should study more.
To indicate something hypothetical (in present or future time).
¡Yo viajaría a la luna mañana (si pudiera)! I would go to the moon tomorrow (if I could)!
This type of situation will be studied below in the section on contrary-to-fact or unreal conditions.

The conditional perfect
Forms. The conditional perfect is formed by using the conditional forms of the helping verb haber with the past (or passive) participle:
yo habría hablado/comido/vivido
tú habrías hablado/comido/vivido
él/ella/usted habría hablado/comido/vivido
nosotros/nosotras habríamos hablado/comido/vivido
vosotros/vosotras habríais hablado/comido/vivido
ellos/ellas/ustedes habrían hablado/comido/vivido
(I would have spoken/eaten/lived)

Usage. It is used primarily to indicate something hypothetical or unreal in past time, in a context where the subjunctive is not required such as in the main clause of a sentence:
Sí, yo habría ido a la luna. Yes, I would have gone to the moon [but they didn't ask me].
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 normally used both in the “if”clause and in the main part of the sentence:
Si ella viene mañana, iremos al cine. If she comes tomorrow [she may actually come], we will go to the movies.
Si nieva mucho, podré esquiar. If it snows a lot [it may really snow], I can ski.
Note that the English versions of the above conditions suggest the indicative by the lack of hypothesis-suggesting words such as “would”, and by not using the past tense to refer to a present-time situation.

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]
Reminders/tips
The present subjunctive is NOT used after si (“if”)!
In unreal conditions the standard pattern is a past subjunctive in the “if”clause and a conditional tense in the main clause:
“if” clause main clause time aspect
si + imperfect subjunctive conditional present/future time actions (but expressed by the past tense in both English and Spanish)
Si hablaras, te creerían.
If you spoke, they would believe you.

si + past perfect subjunctive conditional perfect past time actions (expresed by previous-past time tenses both in English and Spanish)
Si hubieras hablado, te habrían creído.
If you had spoken, they would have believed you.
There are frequent tip-offs in English sentences that the conditional/past subjunctive combination is required in the corresponding Spanish sentence. (Compare with the examples above):
Use of the past tense (“spoke”) in the “if” clause to indicate a present/future situation. (“If you spoke [right now]...”).
Use of the word “would” in the main clause to indicate conjecture for present/future time. (“...they would believe you [now or in the future]”).
Use of “would have” to in either clause to indicate conjecture/hypothesis in past time. (“...they would have believed you”).