Learn French: Ils avaient prévenu l'équipe pour éviter le désastre. - They had warned the team to avoid the disaster.

Avaient prévenu is the Plus-que-parfait (pluperfect) tense, which translates to 'had warned'. It is formed by using the imperfect tense of the helper verb (avaient) plus the past participle (prévenu). You use this tense to show that an action happened further in the past, before another past event. Ont prévenu is the Passé Composé, meaning 'warned' or 'have warned'.

Wait, doesn't prévenir mean 'to prevent'?

It can mean 'to prevent', but here it means 'to warn' or 'to inform'. It is a common false friend! Notice that to express the idea of preventing the disaster, the sentence uses éviter (to avoid). You could also use empêcher to mean 'to prevent' in that context.

Would the past participle prévenu ever need an 's' added to match the plural subject ils?

Not here! Because the helper verb is avoir, the past participle never agrees with the subject. It would only agree with the direct object if the object came before the verb. Since the direct object (l'équipe) comes after avaient prévenu, the past participle stays exactly as it is.

Why is éviter in the infinitive form rather than conjugated?

It directly follows the preposition pour. In French, when you use pour to mean 'in order to' or to show the purpose of an action, it must always be followed by a verb in the infinitive form.