The Gemara, Berachot 8a, states that one should review the weekly parsha by reading the scripture twice and the Targum (an Aramaic translation) once. This mitzvah is known as shnayim mikra v'echad Targum. There are two ways to understand the nature of the specific formulation to read the scripture twice and the Targum once. One can posit that the nature of the mitzvah is to read the parsha three times. The reading of the Targum is the third reading and serves to provide additional insight into the text. Alternatively, one can suggest that the nature of the mitzvah is to review the parsha. A proper review of the parsha entails reading the scripture twice and the Targum once.

These two approaches seem to be the basis of a dispute between Rashi and Rabbeinu Yonah. The Gemara, ibid, states that the requirement of reading the scripture twice and the Targum once applies even to a verse such as "atarot v'divon" (Bamidbar 32:3), a verse containing only names of places. Rashi, Berachot 8b, s.v. V'Afilu, notes that atarot v'divon is unique because there is no Targum (the text of the Targum is the same as the scripture). Tosafot, ad loc., s.v. V'Afilu, deduce that Rashi's opinon is that when there is no Targum one is required to read the scripture three times. Rabbeinu Yonah, Berachot 4b, s.v. V'Afilu, states that the verse of "atarot v'divon" is singled out because there is no Targum Onkelos but there is Targum Yerushalmi on the verse. Since there is Targum Yerushalmi, one would have to read it instead. If there is no Targum at all, perhaps one is only required to read the scripture twice.

Rashi seems to be of the opinion that the nature of shnayim mikra v'echad Targum is to read the parsha three times. Therefore, if there is no Targum, one would have to read the scripture a third time. Rabbeinu Yonah seems to be of the opinion that the nature of shnayim mikra v'echad Targum is to properly review the parsha. If there is Targum on the verse, one must also learn the Targum. However, if there is no Targum available, it might be sufficient to read the scripture twice.

Replacing Targum with the Commentary of Rashi
Rabbeinu Asher, Berachot 1:8, writes that one may not use a vernacular translation in place of the Targum because the Targum is based on Sinaitic tradition. However, one may use the commentary of Rashi in place of the Targum because he explains all of the words that require translation. Semag, Aseh no. 19, argues that Rashi's commentary is better than the Targum. However he notes that Rabbeinu Yitzchak disagrees and maintains that Targum is better because of its Sinaitic tradition. Mishna Berurah, Biur Halacha 285:2, notes that that there is line missing in the text of Semag, and the real reason why Rabbeinu Yitzchak maintains that Targum has an advantage over the commentary of Rashi is because the Targum provides a translation of (almost) every verse. However, there is no commentary of Rashi on many of the verses.

Perhaps the issue of whether it is preferable to use Targum or the commentary of Rashi is contingent on the two approaches presented above. If one assumes that nature of the mitzvah of shnayim mikra v'echad Targum is to read the parsha three times, the Targum serves as a more appropriate method of reading the parsha for the third time because there is Targum on every verse. However, if the nature of the mitzvah is to properly review the parsha, the commentary of Rashi is more appropriate because it provides a more in-depth understanding of the scripture. Shulchan Aruch, Orach Chaim 285:2, writes that a yarei shamayim (a G-d fearing individual) should read the Targum as well as the commentary of Rashi. This is also the conclusion of Mishna Berurah, Biur Halacha, ibid.

Shnayim Mikra V'echad Targum for a Mourner
It is prohibited for a mourner to learn Torah during the shiva period. Nevertheless, he may recite portions of Torah that are part of the daily routine. Rabbeinu Asher, Moed Katan 3:28, writes that although it is prohibited for a mourner to learn Torah on Shabbat, shnayim mikra v'echad Targum is permitted. He compares shnayim mikra v'echad Targum to the recitation of k’riat shema which is permitted for a mourner. R. Avraham I. Kook, Orach Mishpat, Orach Chaim no. 60, questions the comparison of shnayim mikra v'echad Targum to the recitation of k’riat shema. Shnayim mikra v'echad Targum is primarily considered a form of learning Torah. The mitzvah of k’riat Shema is an independent mitzvah that contains in it an element of learning Torah. R. Kook concludes that Rabbeinu Asher is of the opinion that the mitzvah of k’riat Shema is also primarily a mitzvah of Talmud Torah. Therefore, the comparison of shnayim mikra v'echad Targum to k’riat shema is appropriate.

Perhaps one can suggest that the comparison is valid even if k’riat shema is not primarily a mitzvah of Talmud Torah. If one assumes that the nature of the mitzvah of shnayim mikra v'echad Targum is to read the parsha three times, one can challenge R. Kook's question. Shnayim mikra v'echad Targum is comparable to k’riat shema in that they are both primarily mitzvot of recitation that have a Talmud Torah component to them. Although in both situations one does fulfill the mitzvah of Talmud Torah, the ma'aseh hamitzvah (the action one performs in order to fulfill the mitzvah) is one of recitation, and not one of learning.

Shnayim Mikra V'echad Targum as a Preparation for K'riat HaTorah
Terumat HaDeshen, no. 23, suggests that Rabbeinu Chananel's position (see Rabbeinu Chananel, Berachot 9b, s.v. Rav Bibi) is that the purpose of shnayim mikra v'echad Targum is to enable the congregation to properly understand the Torah reading when it is read publicly. This position does not have to be viewed as a third approach to the nature of shnayim mikra v'echad Targum. Rather, one can understand that Rabbeinu Chananel follows the approach that the nature of shnayim mikra v'echad Targum is a proper review of the parsha. Rabbeinu Chananel's position is an extreme form of this approach in requiring that the review be performed in preparation for the Torah reading.

Terumat HaDeshen adds that according to this position, one would be required to prepare the Torah readings of Yom Tov in the same manner in which one prepares the weekly parsha. He concludes that despite this position, common practice is to complete shnayim mikra v'echad Targum for the weekly parsha and not for Yom Tov.

One can still question why shnayim mikra v'echad Targum is not performed on Yom Tov. Perhaps it is because Rabbeinu Chananel's position is fundamentally rejected. Alternatively, one can suggest that it is not rejected. The reason why shnayim mikra v'echad Targum is not performed on Yom Tov is because one who has performed shnayim mikra v'echad Targum on a given portion is considered adequately prepared for that portion for an entire year. The shnayim mikra v'echad Targum that one reads on the week that the parsha is read will also serve as the shnayim mikra v'echad Targum for that parsha if and when it is read on Yom Tov.

The indicator of whether Rabbeinu Chananel's opinion is accepted lies in the proper time to complete shnayim mikra v'echad Targum. According to Rabbeinu Chananel, one would have to complete the entire parsha prior to the Torah reading in order be adequately prepared for the Torah reading. Shulchan Aruch, Orach Chaim 285:4, rules that one should ideally complete shnayim mikra v'echad Targum prior to eating the Shabbat morning meal. Apparently, there is no particular reason to complete shnayim mikra v'echad Targum prior to k'riat haTorah. This indicates that Shulchan Aruch rejects the opinion of Rabbeinu Chananel. However, Magen Avraham 285:6, rules that one should complete shnayim mikra v'echad Targum prior to Shabbat. Mishna Berurah 285:9, writes that if one cannot complete shnayim mikra v'echad Targum prior to Shabbat, one should try to complete it before the Shacharit services. [Mishna Berurah quotes two sources that one should complete shnayim mikra v'echad Targum prior to the Shacharit services. The first, Or Zarua, Hilchot Shabbat no. 42, states that the reason is because it is difficult to complete shnayim mikra v'echad Targum after the prayer services. The second, Rabbeinu Yonah, Berachot 4b, s.v. L’Olam, states that one should complete shnayim mikra v'echad Targum prior to k’riat haTorah, implying that he subscribes to Rabbeinu Chananel’s opinion.]