England displayed their championship credentials in a 51-7 win over Ireland, but it was France who grabbed the headlines with a 52-3 thrashing of Wales.
"The French pack are heavy, really physical and they know their stuff," Bern, who scored one of the eight tries against Ireland, told BBC's Rugby Union Weekly.
"They showed against Wales they are going to be attacking those breakdowns, so if we make an error, they will definitely score from it.
"We know if anything goes wrong at our end they will take advantage and they have the same amount of passion."
While France's players are still on part-time contracts, Bern said the new fully-professional England team are boosted by extended time for recuperation between games.
"We've only been full time for a couple of weeks but our rest and recovery is probably the biggest change we've had," Bern added.
"Players are now allowed to have that day off where originally they would've had to go back to work."
(Reporting by Rohith Nair in Bengaluru; Editing by Christian Radnedge)
