Frustrated about Translation

I don’t really understand how you can tell when not to translate verbatim? Is there a reason “ Now I cannot stop thinking about her” cannot be translated as “Ahora no puedo dejar de pensar en ella “. I am not seeing why it is translated as “ahora no dejo de pensar en ella”. Why wouldn’t the translation for that just simply be “now I don’t stop thinking about her”. Please help me understand why this is incorrect. These examples are the only reason I am doing poorly. Thank you.