Watch: To look and pay attention to something that is happening or moving. Eg: Priya watched live match at stadium.
Watch somebody do(ing) something means to be careful about something. Eg: Anna need to watch her weight. Eg: She watched them looting a shop.
Watch it: Used to warn someone to be careful. Eg: Hey, watch it you nearly hit my head.
To take care of someone or something for a short time and make sure they are safe. Eg: Can watch my belongings?
Watch out (phrasal verb) means to be careful and pay attention. Eg: Watch out! you may fall.
Watch out for means to be careful. Eg: You can do your driving practise here, but watch out for cars.
Keep a (close) watch on means to carefully watch and pay attention to something which you always know what is happening and are ready to deal with. Eg: Don't worry I will keep a watch on him.