Swansea boss Garry Monk is adamant that only an “astronomical” bid could prise Wilfried Bony away from the club.
Everton manager Roberto Martinez hopes to complete the signing of midfielder Muhamed Besic in the near future.
The 18-year-old Norway youth international has put pen to paper on a deal until June 2016 and the Terriers have the option of an extra year.
Town manager Mark Robins told the club's official website: "As part of our scouting activity in Scandinavia we watched Sondre on a number of occasions for IK Start as well as on international duty with Norway before we invited him to spend a week with us in late November 2013 after his season and contract ended.
"He impressed me and our staff during that training period. He has great technical ability and a superb passing range.
"He's a central midfielder and is able to play as a deep-lying player or as a more advanced 'number 10'.
"Sondre is only 18 and we believe can develop further with us. He has a real desire to learn and do well, so we're excited about working with him over the coming weeks and months.
"We are building a group of young players, through a balance of careful acquisition and developing our academy, which is getting stronger, with lots of potential and incorporating types that fit the long-term philosophy of the club."
Tronstad made his debut for IK Start at 16 and has made a total of 23 appearances, including nine since they won promotion to the Norwegian top flight last season.