Imparfait or ´êtait en train dé'

Bonjour Meghna !

"être en train de" is a way to emphasise the fact of being in the process of doing something, a bit like "I was in the middle of ...", whereas l'Imparfait is closer to expressing "I was -ING".

I hope that's helpful!
Bonnes Fêtes et à bientôt !

Thank you Aurélie. I am still struggling with the nuances- they are difficult to grasp.

In another question - ´What would be the best way to say "I knocked at the door, but you were sleeping."?´ - I see that imparfait usage was marked incorrect. En train de was expected. Seems to be contradicting that '...ING' is imparfait.
Maybe it you can provide the English statement for the example where imparfait would be correct?

Thank you for your patience

Clarification on my query above - I had read the choices incorrectly.

The correct answer is ´J'ai frappé à la porte, mais tu étais en train de dormir.´. Could I say this as j'ai frappé ..... mais tu dormais?