The foundational source is the Gemara in Berakhot 8b, which teaches that one must complete his parshiyos with the congregation, reading each verse twice in the Hebrew and once in the Aramaic targum, even a verse like Atarot and Divon that consists only of place names, and that one who does so merits长 days and years; the Gemara also establishes that this reading must not precede or follow the congregation's own timetable.
The Tur and later codifiers translate this into practical halacha: the Tur (Orach Chayim 285), Beit Yosef (Orach Chayim 285:1), and Shulchan Arukh (Orach Chayim 285:1-3) all rule that every person is obligated to read the week's parasha twice in the Mikra and once in the Targum even though he hears the entire Torah read publicly each Shabbat, and that one who studied the parasha with Rashi's commentary is considered to have fulfilled the targum requirement, with the especially God-fearing reading both Rashi and the Targum. Shulchan Arukh HaRav (Orach Chayim 285:1) and Kitzur Shulchan Arukh (72:11) affirm the same obligation, specifying that a verse lacking a targum must be read three times, and that the mitzvah in its optimal form is performed on Erev Shabbat afternoon.
Arukh HaShulchan (Orach Chaim 285:1-2) and Tiferet Yisrael (13:7) likewise record the obligation and explain the targum's role as rendering the parasha into language the common person understands, since the targum was composed in a tongue once universally known.
There is a dispute among the Acharonim over the correct method of reading: Mishnah Berurah (285:2-4) records that some hold one should read each verse twice followed immediately by its targum, while others hold one should read an entire open or closed section twice and only then its targum, noting that the Vilna Gaon followed the latter method; Rav Eliezer Melamed (Peninei Halakhah, Shabbat 5:10) confirms both customs are valid, attributing the verse-by-verse method to the Arizal and the section-by-section method to the Shelah and the Gra.
Rav Eliezer Melamed (Peninei Halakhah, Shabbat 5:9) explains the historical background of the enactment, noting that it was instituted when the Jewish masses spoke Aramaic and understood Onkelos's translation, and that once Aramaic ceased to be commonly understood, a question arose whether one could substitute a translation into the vernacular or Rashi's commentary in its place.
Among contemporary poskim, Rav Mordechai Eliyahu (Maamar Mordechai Hilchot Shabbat, Chelek 1 2:7), Maamar Mordechai Hilchot Shabbat, Chelek 1 2:2, Maamar Mordechai Hilchot Shabbat, Chelek 1 2:8, Shut HaRav HaRashi 21:3, Maamar Mordechai Hilchot Shabbat, Chelek 1 2:14, Maamar Mordechai Hilchot Shabbat, Chelek 1 2:43, and Maamar Mordechai Hilchot Shabbat, Chelek 1 2:5 together elaborate that the obligation applies equally to laymen and Torah scholars, that it is distinct from the separate obligation to hear the public Torah reading, that it is best performed on Friday morning wrapped in tallit and tefillin though it may be done any day of the week, and that one may enhance the mitzvah over the years by adding additional commentaries such as the Ibn Ezra and the Sforno alongside Rashi.
Kitzur Shulchan Arukh Yalkut Yosef (Orach Chaim 285:1) rules that even one whose entire day is devoted to Torah study and halachic writing must still complete shnayim mikra v'echad targum, though he may do so on Friday night if his schedule requires.
Igros Kodesh letter 10305 cites the position that reading the parasha twice in the original followed by a third reading with Rashi, built on the foundation of the Talmud, can be more beneficial than reading the Targum, while concluding that nevertheless a God-fearing person should read both the Targum and Rashi.
On timing and scope, Responsa Rav Pealim (Volume I, Yoreh Deah 52:1-2) clarifies that the leniency to review a parasha during the days of mourning (shiva) found in Yoreh Deah 400 refers only to someone who did not hear the parasha in shul and must therefore complete it privately in order to keep pace with the congregation, and does not refer to the separate obligation of shnayim mikra v'echad targum itself.
Igros Moshe OC 3:98 rules that when reciting the weekly portion twice with its translation, the divine names must be pronounced in accordance with the translation, and that omitting them, or even omitting a single word, invalidates that recitation of the targum.
Among the poskim addressing the practical case directly, Teshuvos V'hanhagos (3:98) holds that reading the parasha twice with its translation is an absolute personal obligation to be fulfilled together with the congregation's timetable, adding that the ideal practice is to read the text three times and hear it a fourth.
Revavos Ephraim (5:216) records that Rabbi Moshe Feinstein would review the parasha twice word-for-word followed by Onkelos and then Rashi and the Ramban, while the Chazon Ish read the Chumash once, then the translation, then the Chumash again for clarity, and laments the general decline in the study of these translations once prized by the Rambam, the Ramban, Rashi, and the Radak.
Shevet HaLevi (7:33) favors a regular practice of studying each weekly portion together with Targum and Rashi, discusses the differing views on proper reading order, and relates that the Chofetz Chaim would read each verse together with its Targum, Rashi, and Ramban.
Igros Moshe (OC 3:40) rules that one must complete shnayim mikra v'echad targum specifically for the weekly Shabbat parshiyot, but is not obligated to repeat the readings assigned for festivals or other non-weekday Torah portions.
