Translation:Ik hoor je, maar ik zie je niet.
How do I know when to write "je" instead of "jij"?
Jij can only be the subject of a sentence.
I put down "Ik hoor je, maar zie je niet." Is that incorrect?
You forgot to put a second "ik" in your sentence. ;)
Your sentence is not wrong in general, but it is for this translation.