The first different ist he Queen. In the poem she just comes to the end and sais one sentence and in the movie she is acting in different scenes. The first scene where she played was already at the beginning when Grendel comes and destroys everything.
The second different and in my opinion the biggest, is after Beowulf kills Grendel. In the poem Beowulf is going to kill Grendels mother. He kills her after a fight, but in the movie she promis him to protect him when he gives her another son. So Beowulf has „sex“ with Grendels mother and leaves the cave and tells everyone he killed her.