In short

The imperfect subjunctive is the past form of the subjunctive. Build it from the ellos preterite: drop -ron and add the -ra (or -se) endings. Its star job is the unreal si-clause: Si tuviera dinero, viajaría.

The rule

There is a single stem for the whole tense, and two endings sets that mean exactly the same thing.

Step 1 - Start from the ellos preterite. Take the third-person plural of the preterite and cut the final -ron. Any irregularity the preterite has is already inside the stem, so there is nothing extra to memorize.

Step 2 - Add the endings. The -ra set and the -se set are interchangeable; -ra dominates in speech, -se sounds more formal or literary. Only the nosotros form takes a written accent (it becomes esdrújula): habláramos, tuviéramos, fuéramos.

Now the six everyday uses:

• 1. Unreal / hypothetical conditions. After si meaning “if (but it is not so)”, use the imperfect subjunctive; the result clause takes the conditional (Lesson 35): Si tuviera tiempo, te ayudaría.
• 2. ojalá + imperfect subjunctive - a wish felt as unlikely or contrary to fact: Ojalá estuviera aquí (“I wish he were here”). Compare ojalá + present subjunctive for a still-open hope.
• 3. como si + imperfect subjunctive - “as if”: Gasta dinero como si fuera millonario.
• 4. Polite, softened requests with querer, poder, deber: Quisiera un café. ¿Pudiera repetirlo? Debieras descansar.
• 5. Sequence of tenses. When the triggering verb is in a past tense or the conditional, the subjunctive in the que-clause shifts to the imperfect subjunctive: Quería que vinieras (present quiere que vengas -> past quería que vinieras).
• 6. Unknown or non-existent antecedent. In a relative clause (Lesson 37) the verb turns subjunctive when the noun is only sought, hypothetical or denied to exist - imperfect subjunctive when the frame is past: Buscábamos un piso que tuviera terraza (any flat with one) against Encontramos el piso que tenía terraza (a specific one, indicative). A negated antecedent - no había nadie / ningún… que - always takes it.

Forms & patterns

Three regular verbs, -ra set (the -se set is identical in meaning: hablase, comiese, viviese…):

The -se set runs in parallel, person for person - hablase, hablases, hablase, hablásemos, hablaseis, hablasen - and the accent falls the same way, so comiésemos, tuviésemos, fuésemos.

Irregular stems (all inherited straight from the preterite):

Examples

Common mistakes

• Si tendría dinero… -> Si tuviera dinero… - never put the conditional after si; the if-clause takes the imperfect subjunctive.
• Si sería rico… -> Si fuera rico… - same rule: “would” (conditional) never follows si.
• hablaramos, tuvieramos -> habláramos, tuviéramos - the nosotros form is esdrújula and must carry a written accent.
• Ojalá tengo más tiempo -> Ojalá tuviera más tiempo - for an unlikely or contrary-to-fact wish use the imperfect subjunctive, not the present.
• Quería que vengas -> Quería que vinieras - after a past-tense trigger the subjunctive must also be past (sequence of tenses).
• dijiera, dijieron -> dijera (from dijeron) - decir drops the i, so there is no i after the j.

Tips

• The whole tense is free if you know the preterite: master the ellos form and just swap -ron for -ra/-se.
• Every preterite irregular stays irregular here and nowhere else adds surprises: tuvieron -> tuviera, fueron -> fuera, hicieron -> hiciera.
• The unreal pair is a matched set: si + imperfect subjunctive <-> conditional. If one half says “would”, the si half needs the imperfect subjunctive.
• quisiera is the everyday polite “I would like” - softer and more courteous than quiero.
• -ra or -se? Pick either and stay consistent in a sentence; in conversation you will hear -ra far more often.