So if I wanted to say “what was once fascinating became mundane and boring” would I use the imperfect or passé composé for “was once”?
#Past tenses
fleet vortex
lone lion
Imperfect
We use the imperfect because here we’re describing a characteristic which tends to be imperfect
We would use the passé composé in characteristics to describe change.
« Il était rouge mais à ce moment-là il est devenu bleu »
Here, we are describing something that used to have the characteristic of being red/having the colour red (imperfect). Something then happened which causes the colour to change from red to blue. Though that change happened in the past, its consequences (i.e., the thing being blue now) remains to this day hence we use the passé composé
pulsar dust
in most cases the imperfect but i could see using the passé composé if you could replace it with "what had once fascinated" without changing the meaning. passé composé generally describes a single moment where something changes, so if you're talking about a single instance of something fascinating someone in such a way that it's like a one-off event rather than an ongoing property of being fascinating, then passé composé conveys that.
usually you can use either one, it just depends on what you're trying to say.