how does grendel approach herot

Grendel's approach is silent, quick and stealth. He had to be careful not to show his shadow. Check out the translated quote from the poem:

"Then out of the night

came the shadow-stalker, stealthy and swift;

the hall-guards were slack, asleep at their posts,

all except one; it was widely understood

that as long as God disallowed it,

the fiend could not bear them to his shadow-bourne."