The imperfect subjunctive: the forms

Take the ellos form of the preterite, drop -ron and add -ra. That is the whole tense:

• Me dijo que viniera. - He/She told me to come.
• Si yo tuviera tiempo, iría. - If I had time, I would go.

The imperfect subjunctive on one screen

One set of endings for all three families. The yo and él, ella forms are the same, so the sentence shows who:

• Mi madre quería que yo comiera más. - My mother wanted me to eat more.
• Yo quería que ella comiera más. - I wanted her to eat more.

The one rule: third-person plural preterite, minus -ron

Start from the ellos preterite, never from the infinitive. Whatever the preterite did, the stem keeps:

That is why comer and vivir look identical here: their preterites already end the same way, in -ieron.

• Ellos vivieron en Madrid. - They lived in Madrid.
• Queríamos que vivieran cerca. - We wanted them to live nearby.

It inherits every preterite irregularity

The imperfect subjunctive has no irregulars of its own. Every odd form is an odd preterite with the endings added:

• Ojalá estuvieras aquí. - I wish you were here.
• Nos pidió que dijéramos la verdad. - He/She asked us to tell the truth.

Ser and ir share a preterite, so they share this tense too. Only the sentence tells you which:

• Si yo fuera rico, viajaría. - If I were rich, I would travel. (ser)
• Queríamos que fueras a la fiesta. - We wanted you to go to the party. (fueras, from ir)

Stem changes and spellings come along too

The -ir stem changers and the verbs that write y in the preterite carry that over as well:

• El médico quería que durmieras más. - The doctor wanted you to sleep more.

The -j- preterites (dijeron, trajeron, condujeron) have no i before the ending, so neither does the subjunctive: dijeran, never "dijieran".