Two tenses to tell a story: the background and the events.
A story in the past uses two tenses. The past continuous paints the background: what was going on. The simple past tells the events: what happened. Click ▶ to hear the sentences of the joke.
was / were + verb-ing
The background: an action in progress at a moment in the past.
was with I, he, she, it · were with you, we, they
Negative: he wasn't sitting · Question: Was he sitting?
verb-ed or irregular verb
The events: finished actions, one after the other. They move the story forward.
Negative: he didn't lose · Question: How did you lose…? (did + base verb)
The past continuous is the long action; the simple past is the short event that interrupts it.
Patrick says it with when: “I was boarding a ship when another sailor cut off me hand with a sword.”