In general, The subjunctive tense is used when the action you're talking about has some degree of uncertainty. The imperfect subjunctive is used for actions that are very unlikely to happen -- or to have occured. The imperfect subjunctive is sometimes called, the past subjunctive - that's because it often talks about things that were unlikely to have happened in the past.
Use this tense when the sentence conveys, were to happen. For example:
You can also use it when telling a story about something in the past that includes something that did not happen. This is prety confusing! Let's look at an example:
Ela pediu ao senhor que falasse com o gerente. In this case this is something that did not happen: She asked the man if he would speak with the manager. But it didn't happen -- the man did not speak with the manager.
Let's stick to some more common and easier to understand examples!
These OFTEN but not always trigger this tense: se, que. The key is to ask yourself: would this, could this happen? If no, it's probably going to need the imperfect subjunctive conjugation.
Notice how in all of these examples the subjunctive verb is followed up with one in the conditional tense. That's because this tense is absolutely PERFECT for saying things like, IF such and such were to happen it WOULD BE whatever.
-ar imperfect subjunctive regular verbs
|I were to say||eu fala||sse|
|you were to say||você fala||sse|
|we were to say||nós falá||ssemos|
|they were to say
(also eles, elas)
-er imperfect subjunctive regular verbs
|I were to lose||eu perde||sse|
|you were to lose||você perde||sse|
|we were to lose||nós perdê||ssemos|
|they were to lose||eles perde||ssem|
-ir imperfect subjunctive regular verbs
|I were to go out||eu saí||sse|
|you were to go out||você saí||sse|
|we were to go out||nós saí||ssemos|
|they were to go out||eles saí||ssem|