Fasting in Ramadan and other acts of worship are not valid unless accompanied by the proper intention and it is not prescribed to utter it out loud.

‘Allahumma inni saim’ is what you should say to the one who wants to insult you or fight you: I am fasting, and my fast prevents me from responding to you in kind, because I am guarding my fast against obscene talk or false speech.

If someone is not certain that dawn has broken, then he may eat until he is certain. If he knows that the Mu’adhdhin gives Adhan ahead of time, or he is not sure whether he is giving it on time or ahead of time, then he may eat until he is certain. However, it is safer for him to stop (eating and drinking) as soon as he hears the Adhan.

If you intend to make up the Ramadan fasts on consecutive days, then you must form the intention for each day according to the majority of scholars.