What does Odysseus instruct his men to do on Helios' island?
In Book X, before Odysseus and his men encounter Scylla and Charybdis and the Sirens, they are warned by both Teiresias, the prophet whom Odysseus encounters in the underworld, and Circe, the Enchantress, that if they reach the island of the sun god Helios (Hyperion) they must not eat the oxen, who are Helios' children:
If you leave these flocks unharmed . . . you may yet after much hardship reach Ithaca; but if you harm them, then I forewarn you of the destruction both of your ship and of your comrades; and even though you may yourself escape, you will return late, in bad plight, after losing all your men.
But after they endure the danger of Scylla and Charybdis and the Sirens, and after Odysseus has told his crew why they should avoid the island of Helios', Odysseus men who, by the way, cannot be depended upon to stay out of trouble (the Lotus-Eaters, for example), plead with Odysseus to give them a chance to land on the island, have their supper, spend the night, and then depart. Of course, they promise to stay away from Helios' oxen.
Odysseus' last caution to his men about Helios' oxen is as clear as it can be:
My friends,’ said I, ‘we have meat and drink in the ship, let us mind, therefore, and not touch the cattle, or we shall suffer for it; for these cattle and sheep belong to the mighty sun, who sees and gives ear to everything.’ And again they promised that they would obey.
Unfortunately, as Odysseus has experienced, his men, being men, are not entirely dependable. In fact, Homer tells us at the beginning of the Odyssey that Odysseus' men are destined to die "by their own folly" before they can reach Ithaca.
After Eurylochus, one of Odysseus' men, convinces the others to eat the oxen, their fate is sealed. When they leave the island, Zeus creates a deadly storm, which destroys the ship and Odysseus' entire crew, and leaves Odysseus fighting for his life at sea.
One of the themes Homer explores in the Odyssey is the folly of men, and that includes Odysseus, by the way, who acts against his own self interest several times--for example, telling the Cyclops Polyphemus his real name--but Odysseus men, in a real sense, are like children who have to be constantly reminded to behave well. The Helios episode, in which the men are warned several times and in emphatic terms to avoid the oxen, is, unfortunately, proof that men are often their own worst enemies.