The Sages forbade eating matza on Erev Pesaĥ, in order to increase our desire to eat it during the Seder and in order to distinguish between matza eaten before Pesaĥ and the matza eaten as a mitzva during the Seder. This prohibition applies even to children who understand the idea that the matza commemorates the Exodus from Egypt. It is permissible to feed matza on Erev Pesaĥ to small children who do not understand this.
The prohibition begins at dawn on the fourteenth of Nisan, though some choose to be stringent and refrain from eating matza from the beginning of Nisan. Others refrain from eating matza thirty days before Pesaĥ. However, the letter of the law only requires one to refrain from eating matza on the fourteenth of Nisan (MB 471:12).
Israeli hospitals and army bases ordinarily destroy ĥametz several days before Pesaĥ, because otherwise there is concern that ĥametz will remain in the kitchens and camps during Pesaĥ. Matza is served so that the soldiers and patients have what to eat, so they should refrain from eating such matzot on Erev Pesaĥ.
The prohibition against eating matza on Erev Pesaĥ includes even small pieces of matza that have been kneaded with wine or oil. Even if such a mixture is baked, as long as the pieces are identifiable as matza, the blessing before eating them is “ha-motzi” and it is forbidden to eat them on Erev Pesaĥ. However, if after being kneaded and baked the pieces are no longer identifiable as matza, their berakha is “mezonot” and they may be eaten on Erev Pesaĥ (implied in Rema 461:2 and MB ad loc. 19-20). Some poskim are more stringent, maintaining that even if the matza is crumbled like matza meal, kneaded with oil or wine, and baked into cake or cookies so that they are no longer identifiable as matza, it is forbidden to eat them. This is because one who eats enough of them to constitute a meal (“kevi’at se’uda”) still must recite the berakha of “ha-motzi.” Thus, it is apparent that they have not yet lost the status of matza, and consequently the prohibition applies to pastries made of matza meal (Gra, Rav Kook, and Ĥazon Ish).
However, all poskim agree that it is permissible to eat matza balls on Erev Pesaĥ, because after being cooked they no longer carry the status of matza. Even if one eats enough of them to constitute a meal, their berakha is “mezonot,” because they are a cooked food, not a baked good (MB ad loc. 20). Moreover, even if one cooks a whole piece of matza the size of a kezayit or more, most poskim maintain that although its berakha is “ha-motzi,” one may eat it on Erev Pesaĥ (as explained below 14:1).