Nands was declared as well as dead by the doctors. They told the viranis that Nands will never come out of coma. Karan did not consumate his mariage with Tanya until 3 yrs later when Tulsi came back from Jail and told Karan that Tanya is not the ayah(Nanny) but his wife and he should give her the rights of a wife. Karan moved on with his life with Tanya accepting that Nands was dead and will never recover from the coma.
The day Nands came out of the coma state was the day Tanya came to know she was pregnant. Karan had to choose between Tanya & Nands. When Nands read his diary that he has someone else in his life now she fainted and was taken to the hospital. The doctors told him that he had to either hide his relationship with Tanya from Nands or tell Nands everything and leave her forever. At that time he took a decission that no relationship matters to him more than Nands. He decided to stay with Nands and Tanya left SN since Nands was trying to get her married.
Karan found Tanya when she was meeting Bhoomi secretly. He used to meet Tanya since he felt guilty and considered her his responsibility, although he told her everytime that he can't give her the rights of being his wife.
before Nands went to jail she came to know about Karan & Tanya and that Tanya was pregnant. She told Tanya to take care of Karan & Bhoomi. And Karan told her clearly that he can never give her the place of Nands in his heart and that he loves only Nandini. Still Tanya stayed with him, didn't she know that Nands will be back after 14yrs, and that Karan will love only Nands.
Since the leap it was shown that he is still waiting for Nands and for him Tanya is just a friend and mother of his kids.