“have” is used with “I”, “you”, “they” and “has” is used with “it”, “she”, and “he”. , the countable and uncountable nouns sometimes can create lots of confusions as to whether to use singular (has) or plural (have). Police, as strange as it might seem, is referred to as plural noun and therefore take “have” as in: The police have arrested a suspect in connection to the recent robberies in this neighborhood. examples:- I have to study well to get good marks in exam . She has just quit her job. You have to train her as to what she has to do. . Good luck .