"I am not hungry, but I am thirsty."

Translation: Nemám hlad, ale mám žízeň.

[4zDt3]

ja nemam hlad, ale ja mam zizen. - - - -What is wrong?

[JanLyko]

Czech frequently omits personal pronouns. Somewhere you can use them at will (in this sentence "Nemám hlad"/"Já nemám hlad"), somewhere it would sound unnatural ("...ale mám žízeň"/"ale já mám žízeň").