There are six progressive tenses. Some grammarians refer to the progressive tense as the progressive aspect of a verb. The progressive tense shows an “ongoingness” of the action denoted by the verb.
The progressive tense is formed by using the verb to be as an auxiliary verb and adding the present participle (-ing) of the verb. Thus, the past-progressive tense is formed by writing the past tense of the verb to be (was or were) and adding the ‑ing verb. Thus:
He was studying when she called.