The imperfect subjunctive is used widely in many different categories of subjunctive usage and will be referred to repeatedly when other subjunctive uses are discussed. It is important, therefore, to become familiar with it. Points to note: [i] it is easy to recognise [ii] it is easy to form, and [iii] all verbs – no exceptions – are formed in the same way.
[1] The present infinitive of the verb:
1st: amāre
2nd: habēre
3rd: vīvere
3rd-iō: capere
4th: audīre
[2] + the personal endings; the only slight difference is that, in the 2nd singular and the 1st and 2nd plural, the /e/ of the original infinitive ending lengthens > /ē/*
amārem
amārēs*
amāret
amārēmus*
amārētis*
amārent
Similarly:
habērem, habērēs etc.
vīverem, vīverēs etc.
caperem, caperēs etc.
audīrem, audīrēs etc.
That system of infinitive + personal ending applies to all verbs, even irregular ones:
eō, īre: go > imperfect subjunctive: īrem, īrēs, īret, īrēmus, īrētis, īrent
sum, esse: be > imperfect subjunctive: essem, essēs, esset, essemus, essetis, essent
possum, posse: be able > imperfect subjunctive: possem, possēs, posset, possēmus, possētis, possent
volō, velle: want > imperfect subjunctive: vellem, vellēs, vellet, vellēmus, vellētis, vellent

