A verb tense that expresses an action or state that was in progress at a past time (equivalent to English 'was/were + -ing').
英語の過去進行形は“I was reading”のように、過去に進行していた動作を示す。
The English past progressive (past continuous) shows an action that was in progress in the past, as in “I was reading.”
昨日の午後三時、彼は寝ていた——これは過去進行形の典型的な用例だ。
At 3 PM yesterday he was sleeping — this is a typical example of the past progressive.
日本語の文法書では、過去進行形の形成とその使い分けが詳しく説明されている。
Grammar books in Japanese explain how the past progressive is formed and how it's used in detail.