How would you say "They were at the opera yesterday"?
"Eles estavam/estiveram na ópera ontem".
How do you know when to use '...para a/o...' vs '...à/ao...'
ir a = to go for a short time
ir para = to go for an extended period of time, to move.
"à ópera" sounds like "ao ópera" in both the male and the female fast voice