The imperfect tense is a secondary tense. The secondary active tenses express action in the past time, and are built upon secondary suffixes. Here is a table of the secondary active suffixes:

In later modules we will also learn about verbs built upon a set of secondary middle endings. These will include imperfect middle/passive indicative verbs. For now, though, we will concentrate on the imperfect active indicative.

Because imperfect verbs are formed on the present tense stem they have the same kind of aspect as present tense verbs: imperfective aspect, i.e., they focus on the process or duration of the action. Because imperfect verbs have a special past time morpheme attached in front of the stem, they express past tense action. This past time morph is usually an ev prefixed to the stem, which is why it is named an epsilon augment. This past time morph only appears in secondary tenses of the indicative mood.

Hence, the imperfect active indicative expresses ongoing or continuous action in the past.

When translating imperfect active indicative verbs, one will usually employ the English past tense of the verb "to be" (i.e., "was" or "were") and the "-ing" verb form, as in the following example:

evdida,skomen
we were teaching

We will employ an analysis using morph slots to describe the formation of the imperfect active indicative. We employed this method on a smaller scale with future tense verbs when we pointed out the characteristic insertion of the future time morph, the sigma infix.

Imperfect active indicative verbs are made up of at least four morph slots: (1) the  augment morph, which consists of the past time morph, the epsilon augment; (2) the lexal morph, with which the definition or meaning of the word is associated (and in the imperfect consists of the present tense stem); (3) the aspect morph, whose slot in this instance is filled with a neutral morph, a connecting vowel (o/e), inserted for fluid pronunciation; and (4) the suffix, which will be one of the secondary active suffixes.

Observe the morph slots that constitute the following imperfect active indicative forms of  lu,w:

Notice that the neutral morph (or connecting vowel) will be o if a mu or nu follows; in all other instances it will be e. In the third singular, the suffix slot is actually null, so the occasional presence of a movable nu is not technically an exception to this pattern.

Most students find it easier to memorize the resultant imperfect active indicative suffixes that are formed from the aspect and suffix morph slots. 

Then they are able to identify imperfect verbs by observing the presence of the epsilon augment, the present tense stem, and one of these imperfect active indicative endings.

I recommend using this method, while keeping in mind the morph slot formation discussed above.

Here is a small stretch of the ancient Appian Way. The imperfect tense expresses ongoing action in the past. 

(1) The descriptive imperfect is used to vividly represent ongoing action in the past, usually within the context of historical narrative. For example, in Luke 17:27 the activities going on during the days of Noah are listed as imperfect verbs:

h;sqion( e;pinon( evga,moun( evgami,zonto
they were eating, drinking, marrying, being given in marriage

(2) The progressive imperfect does more than express continuous past action (contra Black, Learn, §54). It rather pictures prolonged action taking place over a period of time or, as Black correctly states (Still Greek, 106), "emphasizes the duration of the action." In Luke 2:49 it is appropriate that Jesus uses the imperfect verb form, since his parents had been searching for him for three days:

ti, o[ti evzhtei/te, meÈ
Why have you been seeking me?

Doubtless, there is overlap between the descriptive and progressive uses of the imperfect. The progressive, however, appears to emphasize the duration of the action (again quoting Black).

(3) The inceptive imperfect focuses on the beginning of an ongoing past action. For example, notice how one imperfect verb helps sets the stage for the entire Sermon on the Mount (Matt 5:2):

And when he had opened his mouth, he began teaching (evdi,dasken) them...

(4) The iterative imperfect expresses repeated action in the past. This use of the imperfect is translated into English by using verb helpers like "kept on." The disciples' querying activity in Acts 1:6 seems to involve this use of the imperfect:

They kept asking, saying,"Lord, is it at this time you are restoring the kingdom to Israel?"

The customary imperfect, is a specialized kind of iterative action that conforms to the regularity of habit or custom, as when we are told in Mark 15:6 that, during Passover, Pilate "would customarily release" a prisoner requested by the Israelite people.

(5) The conative or tendential imperfect depicts attempted action in the past. For example, in Matt 3:14 John the Baptist "was trying to prevent" Jesus from being baptized.

For more information on the uses of the imperfect, see Black, Still Greek, 106, or another reference work on Greek syntax, such as Brooks and Winbery (90-95) or Perschbacher (298-303).