How do I form an imperfect subjunctive 'if' clause

Firstly, form the imperfect subjunctive. To do this put the verb in the third person of the preterite (comer becomes comieron). Now drop the -ron leaving comie. Then add the Imperfect Subjunctive endings...

-ra, ras, ra, ramos, raís, ran (which are the most common endings 

or -se, -ses, -se, -semos, -seis, -sen

Either can be used to form the Imperfect Subjunctive.

Now to form the sentence, start with the 'si' or the if, implying doubt

This leads you to use the imperfect subjunctive.

Then the next verb in the sentence should be conjugated to the conditional form.

e.g Si fuera rica, compraría mi propia casa.

Si tuviera más tiempo, haría mis deberes. 

It's important to remember that any verb which is irregular in the Preterite, is also irregular in the Imperfect Subjunctive, as shown in the examples. 

But now you know how to form an 'if' clause with the Imperfect Subjunctive