Why does the Sefer begin with the lineage of Bnei Yehudah?
Radak: The Sefer is primarily about Malchei Yehudah.
Why does it say "Nolad" (singular)?
Malbim: Each was born alone, without a twin. According to Seder Olam (2), each was born at the age of eight, so one might have thought that twins were born 1 .
I cannot resolve our text of Seder Olam. Malbim hints to how everything (from Yehudah's marriage until the birth of Chamul) fits in the 22 years that Yosef was away. Our verse teaches that Onan and Shelah were not twins, even though the Torah did not mention a new pregnancy for Shelah. If we allow a year for each pregnancy, boys must have married [or be expected to do Yibum] at the age of eight. This is a Chidush - nowadays, a boy can do Bi'ah only after nine years! Shelah was born after three years. He should have done Yibum at the age of eight (so far, this is 11 years). Tamar waited an extra year afterwards (12) - "va'Yirbu ha'Yamim." Peretz and Zerach were born (13). Peretz fathered Chetzron and Chamul (22) - they were twins, or from different mothers. Had Onan and Shelah been twins, this could reduce the Chidush (raise the age of marriage or Yibum). However, Rashi (Bereishis 38:27, from Bereishis Rabah 85:13) brings that Tamar's pregnancy was less than nine months. If all sons of Yehudah and Peretz were born in the seventh month, the age of marriage or Yibum could be nine or 10! (PF)
What do we learn from "ha'Kena'anis"?
Malbim: Bas Shu'a was "Bas Ish Kena'ani." Here teaches that he was a Kena'ani, and not a merchant.
How was Er "Ra b'Einei Hashem"?
'Rashi': He had Bi'ah Lo k'Darkah [lest Tamar get pregnant, and mar her beauty].
The Torah says that also Onan was bad in Hashem's eyes. Why is this omitted here?
Malbim #1: Onan had a reason - he knew that the child would not be considered his. (Even though Hashem was angry, it was not as bad as Er.) Er's intent was purely evil.
Malbim #2: This Sefer does not write everything [in earlier Seforim].
