Indicative Imperfect vs Imperfect Progressive

[lizaann]

I don’t see a practical difference between the following two examples. According to Duolingo, each requires a different tense, but in both cases, the construction and time sequence look the same. The sentence to translate brought to mind the imperfect, but I was wrong.
1.
Translate: He was giving her the ring when it started raining.
My translation: Él le daba…
Correct translaton: Él le estaba dando…
2.
Translate: When she was going into class, the professor called her.
Correct translation: Cuando ella entraba a la clase, el profesor la llamó.

Both describe a continuing action in the past, interrupted by a shorter action. How do you make a choice without guessing?