What did Grendel's attacks force Hrothgar to realize about himself, and what advice does he give to Beowulf?

O flower of warriors, beware of that trap.
Choose, dear Beowulf, the better part,
eternal rewards. Do not give way to pride.
For a brief while your strength is in bloom
but it fades quickly; and soon there will follow
illness or the sword to lay you low,
or a sudden fire or surge of water
or jabbing blade or javelin from the air
or repellent age. Your piercing eye
will dim and darken; and death will arrive,
dear warrior, to sweep you away.

Hrothgar's advice is simple..... he warns Beowulf to beware of pride and vanity, to keep himself from being distracted by his success to the point that he is unable to see other difficulties looming ahead. In this, Hrothgar is also speaking to the days of prosperity and the building of Heorot, an accomplishment that left him complacent and an easy target for Grendel.