Which tense is characterized by the structure "was/were + verb -ing"?

The tense characterized by the structure "was/were + verb -ing" is indeed the Past Progressive Tense. This tense is used to indicate actions that were ongoing in the past at a specific time or during a particular event. For example, in the sentence, "I was reading when the phone rang," the use of "was reading" showcases an action that was in progress before it was interrupted by another action (the phone ringing).

In this structure, "was" or "were" serves as the auxiliary verb, while the main verb takes on the "-ing" form to convey the ongoing nature of the action in the past. This clearly distinguishes it from the present perfect, future progressive, and present progressive tenses, which have different structures and purposes. The present perfect tense uses "have" or "has" combined with the past participle, the future progressive includes "will be" plus the "-ing" form, and the present progressive employs "am/is/are" followed by the "-ing" form.