why is tanya taking of karan baby

what is happened with nadini, where is she. karan and naidina loved each other than why is karan with tanya, what is wrong

 

 Nandini is in koma due to the doctor's fault during her delivery.Karan had to marry Tanya 4 sake of Bhoomi( ansh-Nandini's daughter)Tanya looks after her.