"I only drink water."
Translation:Piji jen vodu.
This word order means something else than Piji jen vodu.
Piji jen vodu. means that water is the only thing I drink.
Jen piju vodu. would be used "Co to děláš?" "What are you doing?" "Jen piju vodu." "I am just drinking water."
I do not think it is a good translation here with the past simple tense.