How to use the imperfect subjunctive.

The imperfect subjunctive is a tense used in the same contexts that you would use the indicative (or present) subjunctive, yet referring to something in the past.
You compose a phrase using the imperfect subjunctive starting with establishing the verb you want to use in the preterite "they" form, before removing the "on" or "ieron" at the end of the verb, replacing it with the "ara" or "iera" in the correct conjugation.
For example, take the verb "tomar". In the preterite "they" form it would be "tomaron", and then you remove the "on" and replace it with "ara" to make it into "tomara", which is the imperfect subjunctive meaning "I took" or "he/she took".