their, there, they're

If you don't know the differences between their, there, they're, your, you're, its and it's, you lack a fundamental grasp of the English language. I'm not joking. Fortunately, it's not too difficult to learn.

'Their' is the possessive form of 'they.' So if they have a home, you can say it's their home.
'There' refers to a place. The dictionary is there.
'They're' is a conjunction of 'they' and 'are.' They're going to the movies.

Cobra Commander was incensed at the treachery of Destro's Iron Grenadiers. "They're attacking Cobra Island with their G.I. Joe allies," he steamed. Meanwhile, at Joe headquarters: "Cobra defenses are strong here," said Hawk pointing to the south face of the island. He moved his finger to the north beach. "Their forces are weaker there. Now you know." He paused and turned to the camera, "And knowing is half the battle." "Shut up," said Snake-Eyes, uttering his first words in decades.