Ο γάτος. The cat (male)
Η γάτα. The cat (female)
Το γατί. The kitten (neutral)
Οι γάτες. The cats (plural)
So based on gender and whether it is singular or plural, the the is changing. Not only that, if it comes with "for" (for the) it also changes (why? I don't know!). So here is the breakdown:
Για τον γάτος. For the cat (male)
Για την γάτα. For the cat (female)
Για το γατί. For the kitten (neutral)
Για τις γάτες. For the cats (plural)
As usual with Greek, just a case of memorising them. Leave a comment below if there are any typos or mistakes please!