As noted, the Sages ordained eating a vegetable dipped in liquid between kiddush and the recitation of the Hagada in order to change routine; all year long, we eat vegetables during the meal, after washing hands over bread, but at the Seder we eat a bit of vegetable before reciting the Hagada and before washing hands for the meal. Two things are unusual about this: first, we eat a vegetable before the meal, and second, we wash our hands twice instead of once (Rashi and Rashbam on Pesaḥim 114a; Tur §473). In addition, because we eat a vegetable before reciting the Hagada, our Seder meal is imbued with added importance, because the finest banquets generally begin with appetizers and hors d’oeuvres, followed by a pause for a different part of the program, after which the main meal begins (based on Baḥ).
The word “karpas” appears neither in the Mishna nor in the Talmud; we are only told that a vegetable is eaten before the Hagada is recited (Pesaḥim 114-115). But a few Rishonim (Maharil, Raavan) write that karpas should be used, because its Hebrew name alludes to the 600,000 men put to hard labor in Egypt (the Hebrew letters of the word karpas can be rearranged to spell “samekh parekh”; the letter samekh has a numerical value of sixty or 600,000, and parekh means hard labor). Though not mandatory, the Aḥaronim say that it is good to use karpas (SA 473:6, MB 19 and Kaf Ha-ḥayim 49 ad loc.). However, there are differing opinions about what karpas is. Some say it is celery, and this is the widespread custom among Sephardim. Others say it is parsley, which is the custom of some Ashkenazim. Most Ashkenazim, however, use neither celery nor parsley, because according to Ashkenazic custom, there is uncertainty about what blessing to recite over them. They instead use boiled potatoes. Each family should continue its own tradition.
We dip the karpas in salt water or vinegar and recite the berakha of “borei pri ha-adama” (Who creates the food of the soil) with the intention that it also apply to the maror that will be eaten later in the meal. It is not necessary to recline while eating karpas because some poskim say it alludes to the suffering of enslavement in Egypt, and therefore need not be eaten as a demonstration of freedom.
It is proper to eat less than a kezayit of karpas. Though some Rishonim (Rambam) say that more than a kezayit of karpas should be eaten, it is best to avoid this. Eating more than a kezayit invites uncertainty about making a berakha aḥarona, since according to Ri, a berakha aḥarona is necessary, but according to Rashbam, one should not recite a berakha aḥarona, because the blessing over the karpas covers the maror we eat during the meal. Therefore, as said, it is best not to eat a kezayit of karpas. If one eats more than a kezayit of karpas, he should not recite a berakha aḥarona, because we rule leniently whenever there is uncertainty about reciting a berakha (Maharil; SA 473:6).
. This dispute hinges on whether one must recite the berakha of “ha-adama” on maror. According to Rashbam, a berakha is necessary, since the berakha of “ha-motzi” over the matza covers everything that will be eaten as part of the meal, meaning anything that is eaten with matza to provide satisfaction. But maror is not eaten for this purpose, so it is not covered by “ha-motzi.” Therefore, one must have in mind when making “ha-adama” on the karpas to cover the maror as well. By doing this, he exempts himself from making a “ha-adama” on the maror. However, according to Ri, one need not make a “ha-adama” on maror at all, since it is considered a food that is eaten in the context of the meal and hence is covered by the “ha-motzi” on the matza. Accordingly, if one eats more than a kezayit of karpas, he must recite a berakha aḥarona. He should not wait until Birkat Ha-mazon, since it only covers food that was eaten during the meal, not before it. So if one eats a kezayit of karpas and does not recite a berakha aḥarona right away, he misses his opportunity to make the berakha.
Nevertheless, be-di’avad, if one ate more than a kezayit of karpas, he should not recite a berakha aḥarona. This is because we are lenient in laws of berakhot in cases of uncertainty. On the one hand, perhaps Rashbam is correct that the berakha on the karpas covers the maror, and since Birkat Ha-mazon covers the maror, it also covers the karpas, which is connected to the maror. (See BHL 473:6, which cites Gra that one must make a berakha and concludes that the matter must be explored further. Nevertheless, practically speaking, one should not recite a berakha about which there is doubt.) On the other hand, if one ate a kezayit and recited a berakha aḥarona afterward, he should not recite a berakha on the maror, since perhaps Ri is correct that the berakha on the matza covers the maror.
The halakha that one must eat less than a kezayit of karpas raises a problem: Why is one required to wash his hands? We learned (SA 158:3; Peninei Halakha: Berakhot 2:6) that if one eats less than a kezayit, even of bread, then according to many poskim one need not wash his hands – so certainly one need not wash for a food dipped in liquid. Indeed, according to Rambam (MT Laws of Ḥametz and Matza 8:2) and many other Rishonim, one should specifically eat a kezayit of karpas (in a responsum he instructed his correspondent to recite a berakha aḥarona afterward). The Vilna Gaon and several other Aḥaronim had the custom to eat a kezayit of karpas (although it is uncertain whether they recited a berakha aḥarona afterward). BHL 473:6 leaves this issue unresolved. Perhaps according to Netziv (cited above in n. 12), who says that we wash our hands before karpas to remember what used to be done in the Temple, we can suggest that in the times of the Temple people would eat pieces larger than a kezayit, and they knew whether or not to recite a berakha aḥarona. Nowadays, however, since we do not know whether to recite a berakha aḥarona, we eat less than a kezayit to avoid uncertainty, and although technically we need not wash our hands to eat less than a kezayit, we do so anyway to commemorate what was done in the Temple. (Some say that for foods dipped in liquids one washes on even less than a kezayit, citing the law of karpas as proof. See Kaf Ha-ḥayim 158:20.)