Siempre fuiste una buena cocinera.

English Translation

You always were a good cook.

Preterite is for a fixed time event. The word siempre makes the event very hazy and without a start or stop. Why is this not imperfect?

A fixed-time event can also be a lifetime, for example, if you live in one place all your life.

To me the imperfect doesn’t make much sense, because it describes repeated past actions (you don’t stop and restart being a cook) or an action that used to happen. e.g. “I used to ride motorbikes”. Saying you used to be a good cook, implies that now you’re no longer a good cook. To my mind, the sentence is describing a situation where the speaker has just eaten a meal that the cook has prepared and is trying to flatter her with some praise.

This definitely helps my understanding, thanks