even before our couple joined FO2, they were very good friends..from the army episode, we know that yoona knows about taec's decision to give up his US citizenship to join the army cuz she teased him about it when they were all in the truck...even good friends will share such info...

at that time, both already had certain attraction for each other but didn't act on it..as the female party, yoona will definitely keep it to herself..girls are usually like that..only after the guy reveals...

Taec smash yoona bday cake

Yoona & Taecyeon