Aap/ Tum

Okay since I started to watch the show just recently, meaning from the time Viren climbed on the latter and sunk into Jeevika's room after their engagement I have noticed that Viren is sometimes calling Jeevika tum and sometimes aap. At first I thought maybe it was when everyone else is around he calls her aap and than when they are alone he calls her tum, but watching yesterdays episode I notice that he calls her aap even when they are alone. Not that its a big deal but I find it weird in general when a husband calls his wife aap.