*현재진행시제: am/is/are V-ing

*과거진행시제: was/were V-ing

->V-ing는 동사의 기능을 잃었으므로 be동사 필요

과거진행시제 의미: ~하는 중이었다

->과거의 특정한 시점에 진행중이었던 일

예를 들어 2:30~3:30 수면중이었으면, 아래와 같이 표현 가능

I was sleeping at 3 o'clock yesterday.

추가예문)

I was taking a shower when Lisa called.

Ted was studying in London in 2001.

My parents were working in the same company when they first met.