How do I form the imperfect subjunctive?

Forming the imperfect subjunctive is easy, on the condition that you know how to form the preterite (in 3^rd person plural, e.g. hablaron, comieron…)

Let’s start with a regular verb: hablar.

We go to the 3^rd person plural preterite, (hablaron) and remove the “on”.

From that we have the stem (the last vowel of which is always stressed) to which we can add the endings, which are in order: a, as, a, amos, ais, an.

This yields the following conjugations:

Yo hablara

Tú hablaras

Él/Ella/Usted hablara

Nosotros habláramos

Vosotros hablarais

Ellos/Ustedes hablaran

Similarly, for er/ir verbs like comer (comieron):

Yo comiera

Tú comieras

Él/Ella/Usted comiera

Nosotros comiéramos

Vosotros comierais

Ellos comieran

The accent on these is extremely important; you don’t want hablara to be confused with hablará (future).

One popular way of avoiding confusion is by using the alternative conjugations (which are just as acceptable and grammatically correct). They are also formed from the 3^rd person preterite but have different endings:

Yo hablase/comiese

Tú hablases/comieses

Él/Ella/Usted hablase/comiese

Nosotros hablásemos/comiésemos

Vosotros hablaseis/comieseis

Ellos hablasen/comiesen

Feel free to use whichever conjugation you want, but I find, the second (ase/iese) one is better to start out with as it helps to eliminate confusion (both between tenses and between other words- e.g. the word “fuera” means outside but could also be the imperfect subjunctive of ser or ir but “fuese” can only be the verb).

Finally, remember your irregular verbs; if it’s irregular in the preterite then it’s irregular (in the same way) in the imperfect subjunctive. E.g. traducir goes to tradujeron goes to tradujera,tradujésemos...