The imperfect tense generally expresses an event or state in the past which is "ongoing" or "non-delimited" in time. It generally covers the sense of both English was/were ...ing and used to .... The imperfect is often used to "set the scene", whereas the preterite describes "delimited" actions that took place during or at a specified time. Particularly common imperfect forms thus include estaba (he/she/it was) and estaban (they were).
In Spanish, the imperfect tense takes on one of two forms: -ar verbs have a form based on the suffix -aba, whereas other verbs have a form based on the suffix -ía.
The stressed syllable is always "anchored" on the beginning of the imperfect suffix (so pensaba is pronounced with the stress on the first syllable of -aba).
Irregular imperfect tense forms
Two verbs do not have an imperfect tense based on the above suffixes. The verb ser has imperfect forms era, eras etc (see the full conjugation of ser). The verb ir has a hybrid form iba, ibas etc (see the full conjugation of ir).
The verb ver has forms veía, veías etc (see the full conjugation of ver).