The Gemoro on Rosh Hashono 28b discusses kavonno for Mitzvos and Bal Tosif. From the Gemoro, if a person has explicit kavono to fulfil a Mitzvoh, but it is not now the time for the Mitzva (for example, kavono to sit in the sukka after sukkos), he has transgressed the aveira of Bal Tosif.
Dayan G. Krausz (Rosh Manchester Beis Din) pointed out practical examples of this concerning Kerias Shema of Maariv and Shacharis, if (as is probably more common in Chutz Lo'oretz) one davens Ma'ariv before nacht or Shacharis late. Bearing in mind that we say Mitzvos Tzerichos Kavonno (one should have intention to fulfil a mitzvoh, especially with Mitzvos De'oraisa), and that it is preferable to say Kerias Shema with the brochos, we can present 3 cases:
1) Some who davens Ma'ariv between shkia (sunset) and nacht. Since some hold that it is now the zman for Kerias Shema (and since he is now saying the brochos), he wants to be Yotzei Shema (which will require explicit Kavono). However, according to those who say it is not yet the zman - he will transgress Bal Tosif!!
Solution: Make a tenai ("if I can now be yotzei..., then what I am saying is to fulfil the mitzva; if I cannot now be yotzei, then what I am saying is Limud Hatorah").
Later on (after nacht), lich'ora he can just have kavono to be yotzei (without a tenai), since even if he was already yotzei, he is doing the mitzvah twice (OK according to Tosfos earlier, and OK according to other rishonim since has has a good reason for doing the mitzva again, to be yotzei according to all).
2) One who davens Shacharis between the times of Magen Avrohom and the Vilna Gaon (GR"A), but he wants to say Shema before davenning to be yotzei Magen Avrohom. Now, if one wants the possibility of being yotzei later (in order to say with Shema with Brochos), he should make a tenai before davenning. However, when the says shema again, and he has in mind to be yotzei (to gain the Brochos) - according to the Mogen Avrohom this is Bal Tosif, so he must make a tenai again.
3) One who davens Shacharis after time of Vilna Gaon must say Shema before davenning to be yotei, and later on should not have kavono to be yotzei, as would be Bal Tosif according to all.
I would be interested in comments on this.
Kol Tuv,
Meir Eliezer Bergman, Manchester, UK
This is another important insight from the Talmidei Chachomim in Manchester. First of all I should make it clear that the aim of our Kollel is not to deal with practical Halachah. However I can suggest a few sources for this subject.
(1) To start off with I would like to point out that the whole idea of tenaim in Mitzvos would appear at first sight to be inconsistent with a Gemara we learnt not all that long ago in the Dafyomi on Kesubos 74a. The Gemara states there that since we derive the concept of tenaim from the sons of Gad and Reuven (see Mishnah Kidushin 61a) it follows that a tnai is only valid if it can be performed through a shaliach, in the same way that Moshe Rabeinu commanded Yehoshua to give the sons of Gad and the sons of Reuven a portion in Eretz Yisroel (see Bamidbar 32:28). According to this, since I cannot ask somebody else to say Kerias Shema for me (because this is a "Mitzva she-b'Gufo" - a Mitzva that every individual has to perform himelf) therefore it should follow that I cannot make tenaim concerning my Mitzva of Kerias Shema.
(2) However according to Chidushei Ramban on Bava Basra 126b we can understand how tenaim are applicable for Mitzvos also. Ramban writes that when we learn from the sons of Gad and Reuven that a tenai that cannot be done through an agent is not a tenai, this is only referring to conditions made between fellow-men. In contrast if a person makes a tenai with himself, then it is valid even if it could not be done through a shaliach. Ramban expalins that the reason for this is because the person made the tenai of his own free will, and he wants to fulfil the tenai.
(3) Back to the tenai for saying the shema. This is actually mentioned in Poskim. See the Rema at the end of Shulchan Arukh Orach Chaim siman 46, who writes that every morning, after the Shema that we say before korbonos and pesukei d'zimra, it is good also to say "Boruch Sheim Kevod Malchuso l'Olam va'Ed" because sometimes the congregation daven late and they do not say Shema on time, so one is Yotzei with this shema and boruch sheim etc. that one said before davening.
(4) Rabbi Akiva Eiger writes there that if a person is afraid that the congregation are going to miss the time of shema he should make a tenai:- if they will say it too late then I am yotzei now with the shema before the korbanos, but if they do make it on time, then I will not be yotzei with this. This is similar to someone who counts the Omer with a minyan before nacht, who counts with them without a beracha and makes a tenai that if I remember to count again at home then I am not yotzei now so I can still count at home with a beracha, but if I forget at home then I am yotzei now with my counting in Shul. (see Beis Yosef Orach Chaim #489 DH Kosav R. Dovid Avudraham) However the Gra (in Biur HaGra end siman 46) disagrees with the Rema and writes that it is not correct to be yotzei shema, before pesukei d'zimra, without the brochos on shema.
(5) What I did not understand about what you wrote was that you cited correctly that Tosfos Rosh Hashana 16b DH v'Tokim states that Bal Tosif does not apply if one does the same Mitzva twice. Alternatively the Rashba writes there that if one has a need for doing the Mitzva twice one does not transgress Bal Tosif. If so, in the three cases you give there is not a problem of Bal Tosif at all both according to Tosfos and the Rashba, so what is the whole discussion about Bal Tosif here?
A Gutter Voch,
Dovid Bloom