He was staying in Rome that summer.
He stayed in Rome that summer.
Can I use the two sentences above interchangeably? Or do they have slightly different meanings?
I am a learner myself and I understand your confusion. I agree that the past simple is reporting a fact, but the past continuous can be used to emphasize a temporary or new situation "he was staying in Rome that summer, the rest of the year he stayed in Milan." That could be one reason to use the past continuous. One thing I have learnt is that context is everything.