Why does the Torah insert the (otherwise superfluous) word "va'Tosef la'Ledes"?
Rashi: It teaches us that more children were born. 1
Targum Yonasan: Since we saw in the previous Pasuk that Adam was not the father of Kayin, the Torah needs to write here that 'she added to give birth from her husband' - Hevel.
What are the implications of "es Achiv es Hevel"?
Rashi: With reference to 4:2:1:1, it teaches us - in conjunction with "es Kayin" in the previous Pasuk, that one twin was born with Kayin, and two with Hevel. 1
Targum Yonasan: The Torah writes "es Achiv es Hevel" - and not "es Achiv Hevel" - to teach us that a twin daughter was born together with Hevel. 2
Yevamos 62a: We learn from the two words 'Es' in the current Pasuk ("es Achiv ve'es Hevel") that one twin (girl) was born with Kayin and one with Hevel - and each one married his sister. 3
See Ohr ha'Chayim's second explanation.
See Na'ar Yonasan.
Which Hashem permitted out of Chesed ("Olam Chesed Yibaneh") because there was nobody else to marry. Oznayim la'Torah - The fact that the wife of each one was born together with him strengthened the bond between them and taught them the importance of family life - that husband and wife should be like one.
Why did Chavah call her second son 'Hevel'?
Oznayim la'Torah: She called her first son 'Kayin' because she figured that seeing as Adam was created by Ha'Kadosh-Baruch-Hu from earth, and she was created from one of Adam's limbs, that, when she 'created' him, she was a partner with Ha'Kadosh-Baruch-Hu, but when subsequently Hevel was born in the same manner, she realized that from now on, that is the way babies would be born and that her initial thoughts were baseless - so she called him 'Hevel'.
Why does the Torah first record Hevel's profession, even though Kayin was the older of the two?
Rashi and Seforno: Because in choosing their respectve professions, Hevel was wiser 1 than Kayin, inasmuch as he avoided farming (since the land had been cursed - Rashi). He chose shepherding because it required more Chochmah (Seforno). 2
Moshav Zekenim: Because initially, Kayin served only Hashem, as implied by Chavah naming him because "Kanisi Ish es Hashem". And it was only when he saw Hevel engaging in a profession tha he did so as well. 3
Oznayim la'Torah: The superfluous word "ve'Kayin Hayah Oveid Adamah" implies that Hevel honored his older brother and became a shepherd only after Kayin took up farming - to avoid encroaching on his brother's Parnasah.
QUESTIONS ON RASHI
Rashi writes: "'[Hevel became] a shepherd' - Because the earth had been cursed, he ceased to work it." Why explain in this way?
Gur Aryeh: Why would Hevel have chosen sheep-herding as a source of livelihood, at this point in history? As meat was forbidden, he could benefit only from the milk and the wool! It must be because he wanted to avoid farming the earth.
Rashi writes: "'[Hevel became] a shepherd' - because the earth had been cursed." If so, why did Kayin, the firstborn, choose to work the land anyway?
Gur Aryeh #1: Working the land for bread is crucial to sustain life; man could not survive on milk and wool alone. Our Pasuk means that Hevel now became a shepherd (va'Yehi Hevel); after Kayin, as firstborn, had already taken the primary responsibility of working the land (v'Kayin Hayah). 1
Gur Aryeh #2: Kayin was evil-spirited, and predisposed towards the earth that had been cursed.
Gur Aryeh is noting the difference between the verbs "va'Yehi" and "Hayah." This use of the past tense for Kayin parallels the word "Yada" regarding his birth (see Rashi to 4:1).
