I'm probably making an obvious mistake, but why is "does it open" incorrect?
"Does it open?" would be An osclaíonn sé?
Go raibh maith agat.
oscailte is what is known as the "verbal adjective". So, basically, it's an adjective describing something as "open".
My audio really sounds like 'Google sé oscailte' (!). Very unclear...Anyone else hear this?
Agreed, audio was difficult to hear.
could this also "is he open?" ("open" in the sense of a character trait)
“An bhfuil sé ar oscailt” is better. This means “Is it opened” really - i.e. someone has opened it.
This is probably a dumb question but I'm still lost on when to use sé and when to use é (also sí, í)
Sé is the subject of a verb. É is the subject of the copula, the direct object of a verb, and any other circumstance that calls for a 3rd person singular masculine pronoun.