Imperfect vs perfect

Bonjour Clem,

That's a really good question!

For "Sophie wore a costume", you actually have both options as it depends on the narrator's perspective:

- L'Imparfait to describe a continuous action (wearing a costume during the evening)

or

- Le Passé Composé to express a one-time completed action (the dressing up is completed)

   -> This is the case for the parents too

As for "Grandma was here too", you are describing her presence at the event (i.e. a state of being). In this case, L'Imparfait is the correct tense.

I hope this is helpful.

Bonne journée !