What is the machlokes all about? You could learn that the debate revolves around the question of whether mitzvos tzrichos kavanah or not. Rav Huna holds that since mitzvos tzrichos kavanah, even if you eat maror as karpas, it doesn’t matter, because you have no intent at that moment to fulfill the mitzvah of maror. Later, when you intend to do the mitzvah, you recite the bracha. Rav Chisda holds that mitzvos ain tzrichos kavanah and therefore whether you intend it or not, when you eat maror as karpas, you fulfill the mitzvah of maror and should say the birchas hamitzvah.Tosfos rejects this interpretation. If Rav Chisda’s point was that mitzvos ain tzrichos kavanah, he should have said explicitly that you were yotzei maror already at the time of karpas. Tosfos writes that the machlokes here is about hilchos brachos – can you separate the birchas hamitzvah of maror from the kiyum mitzvah of maror that will take place later? Rav Chisda’s chiddush is that even though you won’t fulfill the mitzvah of maror until later in the seder, you can still recite the birchas hamitzvah on maror earlier at the time of eating karpas. Tosfos draws an analogy: just like we say the bracha on tekiyas shofar on the tekiyos d'meyushav done before musaf even though the main mitzvah is to hear tekiyos al seder habrachos in shmoneh esrei, so too, one can say the bracha on maror when eating karpas even though the mitzvah of maror will only be fulfilled later.
My friend Chaim Markowitz asked: Tosfos' analogy doesn’t match. When you blow tekiyos d’meyushav before musaf, you are fulfilling the mitzvah of tekiyas shofar – the Chachamim told you to blow those extra tekiyos in order to mix up the satan (R”H 16). The Chachamim didn’t tell you to eat maror at the time of karpas. As Tosfos themselves explained, everyone agrees that you are not yotzei maror until you eat it later in the seder. So why should you be allowed to say a birchas hamitzvah on maror if there is no kiyum hamitzvah at all involved?The Chazon Ish (#124) already asked this question and has a different approach to the whole sugya because of it. Let me share with you three possible solutions (of course, there may be more) in what I think is worst to best order:
1) Since Chazal instituted that the mitzvah of shofar should be done al seder habrachos, in the middle of shmoneh esrei, the blowing done before musaf does not count as a kiyum mitzvah of tekiyas shofar. (In some old posts we discussed the chiddush of the PM”G in the Pesicha haKolleles that when the Chachamim tell you to do a mitzvah d’oraysa a certain way, you forfeit not just that kiyum derabbanan if you do it differently, but you also get no credit on a d’oraysa level.) It may be a kiyum of this idea of mixing up the satan, but who says you should be reciting a bracha of tekiyas shofar on that?2) Even though there is no kiyum mitzvah of maror when you eat your maror for the sake of karpas, the ma’aseh mitzvah you are doing is identical to the ma’aseh mitzvah of achilas maror. Tosfos perhaps holds that a bracha can be said on a ma’aseh mitzvah even absent a kiyum.
3) My son’s rebbe, R’ Moshe Brown, dug up a Shibolei haLeket (Hil Rosh haShana #302) that basically echoes Tosfos, but adds a few extra words of explanation: “tibul rishon l’tzorech tibul sheni hu l’heikeira d’tinokos d’kol chovas halayla l’mitzvas haggadah hu…” Why can you say a bracha on maror when you eat karpas? Because, answers the Shibolei haLeket, karpas serves as a means of arousing the kids curiosity and therefore, like everything else done Pesach night, is part of the mitzvah of haggadah. How does the fact that karpas is a kiyum of haggdah help explain why it is a kiyum of maror? It must be, suggests R’ Moshe Brown, that the Shibolei haLeket holds that eating maror itself is part and parcel of the mitzvah of haggadah. By way of analogy, just like saying hallel (as we discussed yesterday) is not an independent mitzvah, but is part of the mitzvah of sipur yetziyas mitzrayim because the mitzvah is not just to tell the story but also to give thanks, so too, the mitzvah is not just to tell the story, but to do so using certain “props” like matzah and maror. Since there is a shared kiyum of haggadah common to both karpas and maror, the birchas hamitzvah of maror is not out of place if said over the karpas.