In my review of Beowulf (2007) I criticize Zemeckis and his writers Gaiman and Avary for altering their source material beyond what is acceptable in translating a literary work to film.
An unfortunate effect of this movie will be to misrepresent the original epic in the minds of people who have never read it or who read it so long ago that they have forgotten the details. Here a a few of the differences.
Hrothgar is a dignified old man and his wife Wealtheow would never dream of spitting on him in public or anywhere else. Nor would she have the option of declining sex with her husband.
When Beowulf comes to Heorot Hall, he and his men must observe a certain protocol and leave their weapons at the door before being admitted to the king's presence. The king would never simply amble out in front of the approaching warriors and shout an informal hello.
Although Grendel has been haunting the hall for many years, the hall is not a wreck. Hrothgar and his people use it during daytime hours and leave it at dusk. The hall is made of wood, not stone.
In the poem, there is no connection between the killing of Grendel and his dam and the later killing of the dragon. Beowulf is still a young man when he goes to the aid of Hrothgar. He takes fourteen men with him. Wiglaf is not one of them. He hasn't been born yet. The travelers would not become lost at sea. Beowulf is described as "a man cunning in the knowledge of the sea." Of the fourteen men who go with Beowulf, only one is slain. The others all get back home safely.
Hrothgar is a Dane. Beowulf is a Swede. In the poem his people are called Geats. (By the way, Geat is pronounced /yay aht/, not /geet/.) Beowulf does not succeed Hrothgar as king of the Danes. He goes back home after killing Grendel's mother. Yes, he really did kill her. And yes, she was a sea hag. And no, he did not carry a glowing drinking horn into her cave. The cave was lit from within with a strange, baleful light.
While we're at it, Grendel was an ogre (dined on people) and he was big, but not so much bigger than Beowulf that he couldn't be wrestled with. Beowulf tore his arm off by sheer muscle power, without the help of chains or doors. And when Momma came back the next night, she killed only one man, taking his body back with her to her lair.
The fight with the dragon occurs in Beowulf's old age, after he has reigned over his people for fifty years. A runaway slave stirs up the dragon by stealing a cup from its horde. Beowulf has a special fireproof shield made. He and eleven men, including Wiglaf, go to fight the dragon. It's the young Wiglaf's first battle. Everyone but Wiglaf runs away. Beowulf kills the dragon, but is bitten and dies of the poisonous saliva. His body is placed on a funeral pyre on a cliff overlooking the sea. Afterwards his people cover the pyre with earth, turning it into a barrow. The poet ends the poem with this eulogy of Beowulf: "among the kings of the world he was the mildest of men and most kindly, most gentle to his people and most eager for praise."