umisliti

1. (usually + da) to imagine; to get into one's head
   Umislio je da ga svi na poslu mrze. — He got it into his head that everyone at work hates him.
   Umislila je da je bolesna. — She imagined she was ill.
2. (umisliti se) to become conceited; to get full of oneself
   Otkad je dobio unapređenje, malo se umislio. — Since he got promoted, he's become a bit full of himself.
   Nemoj da se umisliš zbog jedne pohvale. — Don't let one compliment go to your head.

The imperfective is umišljati (se).