"In the morning, behold she was Leah!" Surely she was Leah during the night as well?
Rashi: Of course she was! At night it seemed that she was Rachel, for Yaakov had given to Rachel Simanim, 1 and Rachel divulged them to Leah.
According to the Midrash, the Simanim were the three Mitzvos for which women [who transgress them] are prone to die when giving birth - Nidah, Chalah and Ner Shabbos.
Why did Yaakov end up marrying two wives?
Maharal (Netzach Yisrael beg. Ch. 34, p. 154): Had there been just one wife, there would have been no divide between the tribes; and such a state is more appropriate for Bnei Yisrael in this world. Refer to 29:25:1.2:1.
The Gemara teaches that Rachel should have been Yaakov's first wife, but Leah preceded her thanks to prayer (Rachamim). What does this mean?
Maharal (Chidushei Agados Vol. 3, p. 125, to Bava Basra 123a): Rachel should have been first (and her son the Bechor), for she was the mainstay of the house. Leah used Rachamim; through the trait of Rachamim, a person can achieve things that he would not have been worthy of otherwise. 1
Maharal (ibid.): Due to Rachel's Tzeni'us (modesty), she later merited the return of the Bechorah. Tzeni'us merits a lofty status, concealed from all others; and this describes Yaakov's level as well. See Maharal (Nesivos Olam, Nesiv ha'Tzeni'us Ch. 1, p. 105).
How did Lavan succeed in swindling Yaakov?
Yaakov had given to Rachel Simanim. He could hardly have known that the righteous Rachel would divulge the Simanim. (EC) Refer to 29:12:1:1**.
Ohr ha'Chayim: Yaakov was careful not to look at her before Bi'ah, lest amidst desire drops of semen exude prematurely. Also, he extinguished the lights before intimacy, 1 like is proper (OC 240:11).
Moshav Zekenim (49:3) brought that there was no need for a lamp in Rachel's tent, due to her [radiating] beauty. How could he think that Leah is Rachel? Perhaps Yaakov did not yet know that Rachel radiated; he had seen her only in daylight, or before she radiated (refer to 29:25:153:3). Tosfos ha'Shalem (28:8) says that Leah was more beautiful than Rachel, but Moshav Zekenim disagrees. (PF)
How was Leah's Kidushin valid? It was mistaken!
Riva: Yaakov suspected that Lavan will give Leah in place of Rachel. He intended, if this is Leah, I am Mekadesh her, lest my Bi'ah be Zenus (outside of marriage).
Tosfos ha'Shalem (1): Kidushin through Bi'ah takes effect even mistakenly.
Chizkuni: After he realized that it was Leah, he had Bi'ah with her again for the sake of Kidushin.
Ner Uziel (p. 174): Yaakov intended for his Zivug. Since he received the Berachos appropriate for Esav, now Leah, the appropriate Zivug for Esav, 1 is also a proper Zivug for Yaakov. The physical identity of his Zivug was a trifling detail, so the Kidushin was not mistaken.
Why did Leah trick Yaakov?
Da'as Zekenim, from Bereishis Rabah 70:19: She told him, your father called "Esav," and you answered him. So you called "Rachel," and I answered you! 1 From then, Yaakov began to hate her.
Rachel was so considerate of her sister, that she managed to give Leah the Simanim, without Leah being aware that Rachel had divulged a secret. 2 By extension, Leah was also unaware that Rachel had been Yaakov's intended mate. 3
R. Uziel Milevsky: She was not defending herself through attacking him. Rather, she explained why she is a proper Zivug for Yaakov! Refer to 29:25:151:4.
Perhaps it seemed that Rachel was merely teaching her the Halachos a Jewish wife would need to know before marriage! Refer to 29:25:1:1*.
Why didn't Yaakov realize through her voice that it is Leah? He was in the house for seven years!
Riva: He was very modest, and did not talk much with his wife (after the Nisu'in).
Bechor Shor: He had not talked much with Rachel during the seven years.
Moshav Zekenim (to 29:14): After a month, he separated from Lavan.
If Yaakov thought that he was having relations with someone else (one of the nine Midos of despised relations), Reuven, who was conceived from this, would be spiritually blemished!
Moshav Zekenim, Ra'avad (in Shitah Mekubetzes Nedarim 20b DH Asnas) #1: Reuven was not conceived that night, 1 rather, after Yaakov knew that it is Leah.
Ra'avad #2: The blemish is only when he intended for Ervah (someone forbidden to him).
Ra'avad #3: The Torah was not yet given, and they were not forbidden in details of Mitzvos. 2
Tosfos ha'Shalem (to 29:31 (4)): Yaakov suspected Lavan of trickery. He intended, if this is Leah, I am Mekadesh her through Bi'ah.
Magen Avraham (240:9): The Isur is only when he sees a woman, intends for her, and has Bi'ah with someone else. Yaakov had Bi'ah with the body for which he intended.
Birkei Yosef (OC 240:4): The blemish is when one was negligent. Yaakov had Simanim; this was Ones.
Yaakov's intent was for his Zivug; he had Bi'ah with what he intended for. (Heard from R. Uziel Milevsky.) Refer to 29:25:151:4.
Moshav Zekenim: To resolve this with "Reishis Oni" (Rashi to Bereishis 49:3; Reuven came from Yaakov's first Bi'ah), we must say that Hashem held back Yaakov's seed that night. Tosfos ha'Shalem (30:22 (2)) - Reuven was from the first seed that could father a child, i.e. after the first Bi'ah. The verses connote that after Yaakov married Rachel and loved her more than Leah, Leah became pregnant.
Since there was no Aveirah, the child is not blemished. (PF)
Ohr ha'Chayim (to 29:23): Due to his mistaken intent, Reuven did not receive kingship, Kehunah and the double portion of a Bechor.
"Mah Zos... v'Lamah Rimisani" implies that Yaakov had another complaint, other than tricking him. What was it?
Ohr ha'Chayim #1: Had Lavan given Leah in place of Rachel openly, by force, Yaakov would have one complaint. Now that he schemed to do so covertly, this also caused spiritual losses due to Yaakov's mistaken intent during intimacy 1 (refer to 29:25:154:5 and the note there). Tzadikim are overly adamant about what pertains to spirituality.
Malbim: Why did you not give to me Rachel, and why did you give to me Leah, whom I do not desire?
Ha'amek Davar: Why did you not give to me Rachel, and why did you trick me and cause me to have Bi'ah with a woman without my knowledge - it was Bi'as Zenus!
Ohr ha'Chayim: Alternatively, Yaakov had decided to keep Leah. Because he married her via trickery, this decreased his love for her. Or, Yaakov decided that he wanted to marry also Rachel, and feared lest Lavan demand more than seven years work for her. Therefore, he said 'why did you trick me'? I would have agreed to marry Leah. Both of them are equally good (Rachel is not worth more).
QUESTIONS ON RASHI
Rashi writes: "In the morning, behold! It was Leah! - But at night she was not [presumed to be] Leah, for Yaakov had given Rachel signs...." Why doesn't Rashi explain simply, that only in the morning Yaakov could see that it was Leah?
Gur Aryeh: If so, it would have been sufficient to write, 'In the morning, he said to Lavan....' "Behold, it was Leah" indicates that Rachel had been given reliable signs, which were Halachically sufficient for Yaakov to presume at night that it was Rachel - and she gave them over to Leah.
Rashi writes: "[Rachel] arose and gave over the signs...." The Midrash says that in the merit of this act, Hashem would later forgo punishing Bnei Yisrael, at the time of the Churban, for their sin of Avodah Zarah. What was the great merit of this act; and what is the connection?
Maharal (Netzach Yisrael beg. Ch. 34, p. 154): Rachel knew that it was more appropriate for Yaakov to marry more than one wife. That way, the tribes would be divided among them; for this world is not yet the place for Bnei Yisrael to be one whole. Rachel knew she ought to be the first and foremost wife; yet she willingly accepted to let Leah go first, setting the stage for Yaakov to marry multiple times. Likewise, Hashem's unity will not be ultimately revealed in this world; rather only in the future. Therefore, Hashem forgave the Bnei Yisrael for the sin of Avodah Zarah, for that is the nature of this world.
Rashi writes that Rachel gave the Simanim to Leah. Why did she help her father to trick Yaakov?
Rashi: It was to spare her sister from shame. (If she does not know the Simanim, Yaakov will say, 'I do not want to marry this woman!')
Ner Uziel (p. 178): Rachel understood through Ru'ach ha'Kodesh that also Leah was Yaakov's Zivug.