The Imperfect tense typically refers to the imperfect aspect in the past time. This is the only place where the imperfect aspect is formally distinguished from the simple aspect (that is, the Present Tense serves both for the Simple Present and the Present Imperfective, as does the Future Tense). This typically narrative tense describes actions as ongoing, continuous, habitual or attempted in the past. (It must be distinguished in your translations from the perfect, which describes completed action.) The imperfect is translated as "I was praising", "I used to praise", "I kept on praising," or "I began to praise".
The imperfect indicative active is formed by finding the present stem (the 2nd principle part less the final "-re"), adding the infix "-ba-", and then adding the personal endings. So:
However, by a process of analogy the Romans began to think of the ending of the imperfect as -ybam ... for every verb conjugation except the first conjugation, which was -nbam. This is how that happened:
For most of Historical Latin, the only two conjugations that Romans used to create new verbs were the first conjugation (-nre) for creating transitive verbs and the second conjugation (-yre) for creating intransitive verbs. These are called "Active Conjugations". The similarity of the second conjugation with its long-e stem, habybam to the third conjugation, with it short-e stem, vivebam, appears to have lengthened the short -e- of vivebam to vivybam, by analogy. Once that happened, it was not much of a step to lengthen the short -e- of the third -io conjugation as well, and so the Romans began saying, capiybam. Inevitably the long -i- of the fourth conjugation, sentibam, became the familiar sentiybam. As a result, it may be useful to think of the imperfect tense as marked by -ybam everywhere except in the first conjugation where the ending is -nbam.
To form the Imperfect active subjunctive, simply add the personal endings on to the present active infinitive (the 2nd principle part): for example
For more information on forms, check the paradigm pages: paradigms