The Rabbis prescribed that we add a special blessing for the fast, called Aneinu, in our prayers. The cantor inserts it in between the blessings of Go’el Yisrael and Refa’einu when he repeats the Shemoneh Esrei of Shacharit and Minchah. He says it only if there are at least six people in the congregation fasting, and he has to be one of them (SA 566:5).
Individuals, however, do not say Aneinu as a separate blessing in their silent prayers. Rather, they insert it in the middle of the blessing of Shomei’a Tefillah (Ta’anit 13b). There are various customs as to when we say Aneinu. Some say that one should recite Aneinu in all three prayers of the day. And even though we do not fast at night, one should say it in Ma’ariv because the day as a whole is called a fast day. Yemenite Jews and some Sefardic Jews follow this custom. Most Sefardim say Aneinu only when the fast is in effect. Therefore, on the minor fasts they say it in Shacharit and Minchah, and on Tish’a B’Av, they say it also in Ma’ariv (based on Razah, KHC 565:17). Ashkenazi Jews are accustomed to saying Aneinu in Minchah alone, because they are concerned that perhaps someone will say it in Shacharit, become weak during the day, and break his fast. Then, his statement “on this day of our fast” will turn out to be a lie. Therefore, they say Aneinu only in Minchah, because one who has fasted this long will probably complete the fast (based on the Geonim and Rashi; Rama 565:3). Everyone should continue his family custom.
One who eats less than an olive-sized portion of food or drinks less than a cheek full of liquid is considered to still be fasting and should say Aneinu. But if one eats or drinks more than that, he has broken his fast and may not recite Aneinu. (Nonetheless, he must continue fasting, as we mentioned above in sec. 6).
If the cantor forgets to say Aneinu and has not yet said God’s name at the end of the blessing of Refa’einu, he should go back and recite Aneinu [in its proper place]. If, however, he has already said God’s name, he should say Aneinu in the blessing of Shomei’a Tefillah, as an individual does. And if he forgets to insert it even there, he should say it, without its conclusion [“Blessed are You, O Lord, Who answers in time of trouble”], after “Blessed are You…Who blesses His nation Israel with peace,” before he says, “May the expression of my mouth …” (MB 119:19). An individual who forgets to say Aneinu in Shomei’a Tefillah should insert it in the supplications that follow the Shemoneh Esrei. See Piskei Teshuvot 565:2.
It is also unclear how much one needs to eat in order to be considered no longer fasting. Regarding Yom Kippur, Chazal say that eating food the size of a dried date in the time it takes to eat a p’ras (half a loaf), or drinking a cheek full of liquid, nullifies the pangs of fasting. However, the Shulchan Aruch (568:1) writes that one who eats an olive-sized portion of food in the time it takes to eat a p’ras has broken his fast, for an olive-size is the standard measure for all [mitzvot or sins related] to eating. See Piskei Teshuvot 568:1, who explains that the Acharonim debate the matter. And since there is doubt regarding this issue, it is better to be passive and not say Aneinu if one ate an olive-size of food.