"I do not understand her."
Translation:Я её не понимаю.
This would change the meaning: «Я не её понима́ю» means 'Whom I understand is not she', 'The person I understand is not her', 'I understand someone else, but not her'.
Just for the sake of completeness, Не я́ её понима́ю. would mean 'The one who understands her is not me', 'Someone else understands her, but I don't'.
that would mean "she does not understand I" (ignoring the wrong form of понимать) as you can see it sounds wrong at all.
я её не понимаю i do not understand her я его не понимаю i do not understand him она меня не понимает she does not understand me она её не понимает she does not understand her