There are two forms of imperfect subjunctive, which are used indistinguishably.

Verb Form 1
Verb endings: a, as, a, amos, ais, an

The first form of the verb is formed by taking the 3rd person plural of the preterite minus on.

For the verb hablar, the third person plural of the preterite is hablaron, removing the on yields hablar. To this stem you add the following endings: a as a amos ais an

yo	hablara	I spoke
tú	hablaras	you spoke
Ud./él/ella	hablara	you/he/she spoke
nosotros (as)	habláramos	we spoke
vosotros (as)	hablarais	you guys spoke
Uds/ellos/ellas	hablaran	you all/they spoke

Verb Form 2
Verb endings: se, ses, se, semos, seis, sen
The second is formed by taking the third person plural form of the preterit (e.g., hablaron), minus -ron from the end.

For the verb hablar, the third person plural of the preterite is hablaron, removing the ron yields habla. To this stem you add the following endings: se, ses, se, semos, seis, sen

yo	hablase	I spoke
tú	hablases	you spoke
Ud./él/ella	hablase	you/he/she spoke
nosotros (as)	hablásemos	we spoke
vosotros (as)	hablaseis	you guys spoke
Uds/ellos/ellas	hablasen	you all/they spoke