How do you form the imperfect subjunctive?

regular forms
take the verb --> form in the 'they' version of the preterite --> remove the ending --> add imperfect subjunctive endings
-ER AND -IR verbs
yo] - iera
tú] -ieras
él/ella/usted] -iera
nosotros] -iéramos
vosotros] -ierais
ellos/ellas/ustedes] -ieran
-AR verbs
yo] -ara
tú] -aras
él/ella/usted] -ara
nosotros] -áramos
vosotros] -arais
ellos/ellas/ustedes] -aran
once you establish the endings, it's just a case of forming it so following the rule above the the verb "terminar"
terminar --> terminaron --> termin --> terminara, terminaras, terminara etc