Grendel has been terrorizing and killing the Danes in Hrothgar's hall for 12 years. It's pretty self-explanatory from there.
Beowulf killed Grendel and he was a Dane, so yes
Hrothgar is the King of the Danes that is helped by Beowulf, But he is descended from Scyld Sceafing. The term Danes come from the term Scyldings or Scyld danes. So the first Ruler of the Danes was Scyld Sceafing. The second was his son Beowulf the Dane.
save Hrothgar and the Danes from the monster
The land of the Danes, so Denmark... but not the same Denmark as today.
Beowulf hears of the troubles in Denmark with Hrothgar's kingdom, so Beowulf comes over to help the king out, along with his closest and chosen companions. ( Spoiler: They do it so that they can end the life of the ogre, Grendel, and restore confidence and hapiness to the kingdom.)
Beowulf owed Hrothgar a favor because Hrothgar helped Beowulf's father in time of need. Additionally, Beowulf is looking to prove himself as a warrior.
Beowulf fights Grendel for glory. Tales of a beast terrorizing the people of Herot (the Danes), bring Beowulf and his warriors to help, however, motives are heavily guided by a thirst for fame and glory.
It was pretty successful. Grendel broke into Heorot at night and killed thirty danes. He drank their blood and brought their head shome as trophies. Not bad for a monster's night out!
Grendel feared the coming battle because he didn't not know how he would perform since he had never done battle like that before. Unknown to most of his opponents, Grendel possessed a charm that guarded him from weapons. There was no charm against hand to hand combat so he was troubled he might not emerge the winner in the conflict.
The monster Grendel is causing havoc in Hrothgar's kingdom in Beowulf. Grendel terrorizes the kingdom by attacking the warriors in Heorot, the mead hall.
Grendel is a monstrous and lonely creature depicted as a descendant of Cain in the epic poem "Beowulf." He is described as a vicious and aggressive being who terrorizes the Danes at night and possesses superhuman strength. Grendel is portrayed as an outcast who is envious of the joy and camaraderie of the humans in Heorot.
Beowulf's father took refuge in Hrothgar's land after a messy murder back at home. He eventually married Hrothgar's daughter, and then came baby Beowulf. So long story short Hrothgar is Beowulf's grandfather.