How do you form the imperfect subjunctive in Spanish?

The imperfect subjunctive in Spanish expresses what the present subjunctive would, but is used when talking about a situation in the past. In order to form it, firstly you would need to take the third person plural of a verb in the preterite. For instance, 'tener' takes 'tuvieron' in the preterite when talking about 'ellos.' Then you would need to take away the 'on' and replace it with the following endings: a, as, a, amos, ais, an. In order to conjugate tener in the first person imperfect subjunctive, you would therefore say 'yo tuviera.' Another series of endings that are also used are: se, ses, se, semos, seis, sen. The ending 'ron' would need to be replaced by these. So you could also say 'yo tuviese' as well as 'yo tuviera' if you want to conjugate tener in the first person imperfect subjunctive.