What rule applies to this word?
exhaust
Group IV. When the root has other forms built on the letter a, the -able form is used.
Group VII. When the word has an immediate -tion form, -ible is used.
Group V. When the root ends in hard cor hard g. -able is used.
Group II. When the root is a full word except for a final e, the forms usually are spelled with -able.
Group VI. When the root is not a full word, -ible is used.
Group VIII. When the root ends in ns, miss, or soft cor gible is used.
Group I. If the root is a full word, the ending is usually -able.
Group III. The suffix -able always follows i.

Both characters are similar in the way they are both outsiders.Grendel is an outcast and Beowulf comes from another land.Beowulf still needs to prove he is a heroe while Grendels is always showing he is to be hated and dreaded.

Whenever Grendel attacks the people in the Kingdom he shows how powerful he is.When Beowulf defeats him he shows his power and also that he is stronger than the evil he has put an end to.Both, the monster and the heroe are motivated to act by different reasons.Grendel is motivated by jealousy, nobody admires his strength, Beowulf is motivated by fame, he wants to be remembered and admired.Grendel has been terrorizong the kingdom whereas Beowulf has come to save it.