where does "they" come from here? is it an implied pronoun in Czech?
Pronouns are usually omitted in Czech. You need to look at the verb endings. In this case, 'they, he, she' are all correct and you would need to rely on the context.
Why is ( I ) incorrect
My bad , thanks for your patience.
If I understand your question, it is because the verb would need to be myslím for "I."
Stale na ni mysli - the ststem showed translation "he thinks about her all the time", but I translated as "he is still thinking about her". How to express this meaning correctly?
"He is still thinking about her" is also accepted.