J'arrivais à déchiffrer ... Je l'ouvrais avec impatience. Why imperfect ?

It's the narrative imperfect. There's a good explanation here, but I don't know if this link will work for you:

https://www.google.co.uk/books/edition/Advanced_French_Grammar/QwijBEaBqtwC?hl=en&gbpv=1&dq=%22narrative+imperfect%22&pg=PA123&printsec=frontcover

If not, try googling "narrative imperfect", "imparfait narratif", "imparfait historique" etc.

Hi Tom,

It seems to me that the narrator is recalling what they "used to do" and that is why the imparfait is appropriate?

This is my take on the text  --  hope you agree?

Jim

I agree with Jim. The narrator is describing a habitual situation in the past, hence the imperfect tense.

You would think so, but this is the story of the standout Christmas of her childhood, her vivid memory of catching sight of and tearing open the best present she ever got. It's almost the point of the tale that the events of that morning have no parallel in her memory.

Hi Tom,

I get your point, but what is going on in this person's mind is an enduring memory of a past event.

How would you want to express this?   Is it a completed action?  

I remain of the opinion that imparfait is appropriate - not for the reason that I first suggested (repeated past action - used to) but to deal with your very valid argument -- enduring memory.

It will be interesting to read the input of others here, but this is how I see it.

Bonne journée

Jim

Thank you Jim. It seems your instinct is correct. I think Alan's link explains why.

Bonne journée !

Tom