As we have learned, a kezayit of matza is about a third of a machine matza, and on the Seder night we must eat four or five pieces of this size. After the berakhot of “ha-motzi” and “al akhilat matza,” we eat two kezeytim: one for “ha-motzi” and one for “akhilat matza,” and in order to fulfill the stringent opinion (Ĥazon Ish), which maintains that a kezayit is twice the accepted size. We eat another kezayit for korekh, and one more for the afikoman, though some practice the stringency of eating two kezeytim for the afikoman: one in memory of the Paschal sacrifice itself, and one in commemoration of the matza that would be eaten with it.
If one finds it difficult to eat a matza and a third (or two-thirds), he should do his best to eat the initial two-thirds of a matza as matzat mitzva, in order to fulfill the mitzva in accordance with all of the different opinions. After this, it is sufficient to eat a fifth of a matza for korekh and another fifth for the afikoman. If even this is difficult, one can eat a third of a matza – a kezayit according to the standard measure – to fulfill the mitzva of eating matza, and recite a berakha over it. In other words, during the entire Seder, he would eat one third of a matza followed by an additional two-fifths.
If one has difficulty chewing the matza – for example, one who has no teeth – he may crumble it up and eat the crumbs (BHL 461:4). If even this is too difficult, he may soak the matza in water before eating it. However, if one boils the matza or soaks it until it dissolves, he does not fulfill the mitzva with it, because it no longer has the taste of matza (SA 461:4, MB 19, 20).
It is forbidden, however, to soak the matza in wine, soup, or any other beverage that has a taste, because some believe that this weakens the taste of the matza. Some maintain that even dipping the matza in such liquids is improper. Nevertheless, a sick or elderly person who cannot eat the matza even if it were soaked in water may soak it in another liquid if it would help, recite a berakha over it, and eat it. But if a normal healthy person eats matza that has been soaked in a liquid, he must eat another kezayit of matza (MB 461:18, SHT 32 and Kaf Ha-ĥayim 47-48 ad loc.).