How do i form the imperfect subjunctive tense?

First of all, it's important to remember that you wont be required to spontaneously conjugate verbs into the imperfect subjunctive at this level! You may have to identify them in texts, but you won't be asked to write them down yourself. However, it is definitely worth learning a couple to slip into your writing or speaking exam to really impress the examiner and hit the high marks for range and accuracy. First of all I'll show you how to form the imperfect subjunctive tense and then I'll give you a few verbs that you can easily use in certain phrases to stand out amongst the crowd. So, to form this tense we take the preterite form of the verb you want to conjugate. For example, the preterite form of the verb hablar is hablaron. (I would ask the student to give me the preterite form of this verb to see if they can work it out themselves.) Next, you remove the 'ron' part of the verb to make the stem, which in the case of hablar is habla. Then, you add the following endings, depending on the subject: the 1st person singular 'I' form ending is 'ra', the 'you' singular form is 'ras', to say 'he/she/it/one' you would add 'ra', for the 'we' form you would add 'ramos', adding an accent to the vowel that comes before the ending (the a in this case), for the 'you' plural form we add 'rais', and finally for the 'they' form you would add 'ran'. (I would write these endings down, connected to the full verb, to show the student what the conjugations would look like.) Now that I've shown you how to conjugate the verb, I will give you certain phrases which include the imperfect subjunctive in order to help you wow the examiner.
Conscious that I've written too much here so I will stop. Hopefully this will give you a good idea of how I would answer this question!