May one eat before the morning prayer?
There is a general prohibition of eating a meal prior to any prayer lest one become engrossed in the meal and forget to pray. With regards to the morning, however, there are additional reasons which are learned from verses in the Torah. One verse (Vayikra 19:26) says “Lo Tochlu Al HaDam” (lit. “You shall not eat over the blood”) which is interpreted as meaning that one should not attend to one’s physical desires, such as eating, before praying for one’s “blood”, or soul. Another verse (Melachim I, 14:9) says “Ve’oti Hishlahta Ahare Gevecha” (lit. “and you have cast Me behind your back”) is expounded as meaning that one attended to one’s personal needs before first acknowledging the source of all one’s needs, Hashem. The word Gavecha can be read as Ge’echa (lit. your haughtiness) and the Gemara (Berachot 10b) says that one who eats or drinks prior to the morning prayer is acting arrogantly. As such, the Shulhan Aruch (O.H. 89:3) says that eating and drinking before praying Shaharit is prohibited.
The Shulhan Aruch, however, goes on to say that water may be consumed prior to praying in the morning. Since water is typically drunk in order to give one strength and is not associated with self-indulgence, it is permitted. Similarly, many Poskim include coffee and tea in the list of permitted drinks because they help dissipate feelings of fatigue, and they also allow adding sugar to make the tea or coffee more palatable. Regarding adding milk to coffee or tea, Rabbi Ovadia Yosef (Yabia Omer, vol. IV, § 11) and Rabbi Ben Zion Abba Shaul (Or Lezion, vol. II, §7:7) do not permit it since such a drink is already considered more of a pleasurable treat than simply a drink to give one strength to pray. Nowadays, however, adding milk to coffee is considered part of the way people drink coffee and indeed, Rabbi Shlomo Zalman Auerbach and Rabbi Yosef Shalom Elyashiv do not consider coffee with milk to be associated with arrogance but rather that this is the normal way that coffee is prepared and consumed.
The Shulhan Aruch (ibid:4) goes on to say that if one is thirsty or hungry to the point that one cannot focus during the prayer, then such a person has the status of an ill person and may eat and drink prior to praying.
Summary: Generally speaking, it is forbidden to eat and drink before praying Shaharit. One may have water, or tea or coffee with milk before Shaharit. If one cannot focus in the prayer unless one eats or drinks, one may do so.