Halacha follows the opinion that "Misvot Serichot Kavana" – meaning, one who performs a Misva must have this purpose in mind in order to fulfill his obligation. Thus, for example, before one begins reciting Shema, he must pause and think for a moment in his mind that he prepares to fulfill the Torah obligation to recite the daily Shema. This applies to both the daytime and nighttime Shema recitations.
In some situations, a person does not have specific intention that he fulfills a Misva, but it is clear from the setting and context that this is his objective. For example, even if a person forgets to specifically think about the Misva before he begins reciting Shema, it is abundantly clear that one who comes to the synagogue, dons Tallit and Tefillin, and recites the prayers from the Siddur has in mind to fulfill the Misva. The Mishna Berura (Rabbi Yisrael Meir Kagan, 1839-1933) cites a number of Halachic authorities who rule that one fulfills his obligation in such a case. Even though one must Le’chatehila (optimally) have specific intention to fulfill the Misva, he nevertheless fulfills his obligation even without this intent if it is clear from context that he performs the given act for this purpose. (The Mishna Berura proves this conclusion from a number of Talmudic passages.) Other examples where this ruling would apply are eating in a Sukka and shaking a Lulab on Sukkot. Even if a person does not have specific intention to fulfill the Misva, it is obvious that he eats in the Sukka or shakes the Lulab for the purpose of the Misva, and he thus fulfills his obligation even without specific intent.
It must be emphasized, however, that this applies only "Be’di’abad" (after the fact). Optimally, one must, indeed, have specific intention before performing a Misva that he performs the given act for the purpose of the Misva.
The Be’ur Halacha (supplementary essays accompanying the Mishna Berura) notes that for many people, it would, indeed, be necessary – even "Be’di’abad" – to have specific intention before reciting the nighttime Shema. It is very common during the summer months for congregations to recite Arbit before sunset. Since the Misva of the nighttime Shema takes effect only at nightfall, those who recite Arbit before sunset do not fulfill their Shema obligation at Arbit; they must repeat Shema later, after dark. For such people, then, reciting Shema during the Arbit service in the synagogue is not necessarily done for the purpose of fulfilling the Misva. Since they frequently recite Arbit before sundown, when they cannot fulfill the Shema obligation, it cannot be assumed that they have in mind to fulfill the Misva of Shema when they recite Shema during Arbit. Therefore, if a person who normally recites Arbit before sundown happens on one occasion to recite Arbit after dark, he does not fulfill his obligation of Shema in Arbit unless he has specific intention to fulfill the Misva. Without specific intention, it cannot be assumed that he recites Shema for this purpose, since he normally recites Shema in Arbit without fulfilling the Misva. In this case, then, one who recites Shema without specific intention would have to repeat Shema.
Summary: One who performs a Misva must first think in his mind that he performs the given act for the purpose of fulfilling the Misva. However, if he did not have this specific intention, he nevertheless fulfills his obligation if it is clear from the setting and context that this was his purpose in performing the act. Thus, for example, one must think before reciting Shema that he does so for the purpose of the Misva, but if he did not have this intention, he nevertheless fulfills the Misva since it is obvious that he recites Shema for this purpose. The exception to this rule is the case of somebody who normally prays Arbit before sundown, when he cannot fulfill the obligation of the nighttime Arbit. If such a person happens one time to recite Arbit after dark, he does not fulfill his Shema obligation in Arbit unless he has specific intention to this effect.
