Breakdown of Le pire était que la pluie avait gâché le pique-nique.
être
to be
avoir
to have
le pique-nique
the picnic
que
that (introducing a clause)
Questions & Answers about Le pire était que la pluie avait gâché le pique-nique.
Why use avait gâché (had ruined) instead of the passé composé (a gâché)?
avait gâché is the plus-que-parfait. We use it here to show that the rain ruined the picnic before the past moment we are describing (when it was the worst). It sets up a timeline in the past: first the rain ruined the picnic, then that fact became 'the worst' part of the situation.
Why use the imparfait (était) instead of the passé composé (a été) for 'was'?
Is le pire related to the word pire (worse)?
Yes! pire means 'worse' (it is the irregular comparative of mauvais, meaning bad). By adding the definite article le, it becomes the superlative 'the worst'. In this sentence, it acts as a noun meaning 'the worst thing'.