Using the Imperfect or Past Subjunctive with Conditional

Talking about hypothetical situations, or situations that are contrary to fact, requires some rather complicated grammar. In Spanish, if you are speaking about hypothetical situations for the present time you might want to use the following pattern:

Si + imperfect subjunctive + conditional

or

Conditional + si + imperfect subjunctive 

• Si yo fuera el presidente ayudaría a los niños de la calle.
  If I were the president I would help the children who live on the street.

• Yo viajaría por todo el mundo si fuera muy rico.
  I would travel around the world if I were really rich.

Another common construction which uses the imperfect subjunctive is the phrase como si (as if) to express a situation that is contrary to fact.

• Hablas como si fueras el presidente de la compañía.
  You speak as if you were the president of the company.

• Actúas como si estuvieras culpable.
  You act as if you were guilty.

The imperfect subjunctive is formed by taking the third person plural of the preterite minus -on.

hablaron hablar-
comieron comier-
vivieron vivier- 

To this stem you add the following endings: -a, -as, -a, -amos, -ais, -an.

hablar- hablara, hablaras, hablara, habláramos, hablarais, hablaran
comier- comiera, comieras, comiera, comiéramos, comierais, comieran
vivier- viviera, vivieras, viviera, viviéramos, vivierais, vivieran

The conditional tense is formed by adding –ía: -ías, -ía, -íamos, -íais, or –ían to the end of the infinitive form of the verb.

hablar- hablaría, hablarías, hablaría, hablaríamos, hablaríais, hablarían
comer- comería, comerías, comerías, comeríamos, comeríais, comerían
vivir- vivir’a, vivir’as, vivir’a, vivir’amos, vivir’ais, vivir’an 

Verbs that have irregular forms in the future are also irregular in the conditional. They use the same stem in the conditional as they did in the future tense.

decir: dir-       venir: vendr-
hacer: har-       tener: tendr-
poder: podr-      saber: sabr-
poner: pondr-     querer: querr-