"The tiger hid when we arrived."
Translation:Le tigre s'est caché quand nous sommes arrivés.
Are you sure? I thought both arrivé and arrivés would be correct. https://www.gymglish.com/en/frantastique/french-translation/on-est-arrives
"Arrivé" is correct if "on" means "someone" and it is not the substitute for "nous".
Since the English sentence has "we", the French sentence can have "on" meaning "nous" and therefore the past participle has to agree in gender and number with the idea of "nous":
- On est arrivés
- On est arrivées