What exactly did Yaakov undertake to do with the stone?
Rashi: Refer to 28:18:3:1 and the note there.
Targum Yonasan: That it will be the cornerstone of the Beis-ha'Mikdash, where all of Yisrael will serve Hashem.
Ha'amek Davar #1: It will be Mekadesh everything around it (just like everything inside the Mikdash was Kadosh).
Ha'amek Davar #2: He will teach Torah there.
What did Yaakov mean when he said that he would take a tenth of all that Hashem gave him, to Hashem?
Rashbam: He undertook to take Ma'aser from animals that he would amass, to offer them up as Olos.
Moshav Zekenim, Hadar Zekenim, Da'as Zekenim: He enacted to take Ma'aser 1 from money. He tithed even his children. Since Efrayim and Menasheh are considered like Reuven and Shimon, it is as if he had 14 sons. Four 2 were Bechoros, which are exempt from Ma'aser. He gave Levi 3 to be Ma'aser (Bereishis Rabah 70:9).
Ha'amek Davar: Stam Ma'aser is to the poor. "Aser A'aserenu Lach" means that it will be for Korbanos to Hashem.
Tosfos ha'Shalem (18, citing R. Efrayim): What is so praiseworthy about returning to Hashem a 10th of what Hashem gives to him? It is even if he will get only through great exertion.
Hadar Zekenim: Perhaps Menasheh was not a Bechor. Moshav Zekenim rejects this. The Torah (Bereishis 41:51) calls him a Bechor! (Perhaps he was Yosef's Bechor, but Asnas had a child beforehand - PF.) Rather, he was a Bechor; Reuven's Bechorah was given to Yosef (Divrei ha'Yamim I 5:1), so there were only four. (We could say simply that one cannot split a person, therefore Yaakov gave one son, but it is better to show how a Tzadik's words recorded in the Torah are fulfilled exactly (refer to 48:6:1:1**). Perhaps a 10th of Yaakov's daughters married Levi, so also they were considered Ma'aser. - PF)
Hadar Zekenim gives ways in which Levi was 10th. R. Chaim Paltiel justifies considering Levi the 10th born; I do not understand why. Even Ma'aser Behemah, which must be the 10th to leave the pen, need not be the 10th born! His answer is even more astounding. (PF)
Why did Yaakov use the double expression, "Aser A'aserenu"?
Ba'al ha'Turim: To teach us that that one may not give more than a fifth (two tenths) of one's money for Tzedakah. 1
Tosfos ha'Shalem (3): He enacted Ma'aser Rishon and Ma'aser Sheni.
Tosfos ha'Shalem (15, citing R. Efrayim): This hints to "tithe (Aser), in order that you will become rich (Tis'asher)" (Shabbos 119a).
Moshav Zekenim: He will tithe even the food he eats every day.
See Ba'al ha'Turim. Ohr ha'Chayim (to 29:1, from Kesuvos 50a) - the tithes are equated. The latter is like the former (i.e. do not say that Ma'aser Sheni is a tenth of the 90% remaining after the first tithe.) (The Halacha, however, is that Ma'aser Sheni / Ani is in fact calculated as a tenth of the remaining 90% (i.e. 9%) - and not a tenth of the original total (Tur YD 331). Yet, in a way it can still be equated to Ma'aser Rishon. Ma'aser Rishon starts at 10%, but after the Terumas Ma'aser is removed and given to the Kohen, the Levi's remaining portion also stands at 9%. See Sefer Hafla'ah to Kesuvos 50a.) (CS)
What logic is it to say, 'If you give me a hundred, I will give you ten back'?
Oznayim la'Torah: The fact that Yaakov acknowledged in advance that whatever he accumulated came from Hashem - in spite of the fact that Hashem generally provides through Derech ha'Teva, and that he made a Neder to follow it up by giving something in acknowledgement, in itself demonstrated the high level of Emunah that Yaakov had attained. 1
See Oznayim la'Torah who elaborates further.
Kedushah does not take effect on something not yet in the world!
Tosfos ha'Shalem (18, citing R. Efrayim): However, a Neder takes effect on it.
"Ha'Even ha'Zos" is feminine. Why does it say "Yihyeh" (masculine)?
Bechor Shor: Its place will be Beis Elokim. ('Makom' is masculine.)
Tosfos ha'Shalem (4, citing R. Efrayim): "Even" can be feminine or masculine.
QUESTIONS ON RASHI
Rashi writes: "And this stone [.... shall be a house of Hashem] - As the Targum translates, 'I shall worship there before Hashem,' and indeed he did so upon his return...." What is Rashi explaining?
Gur Aryeh: "House of Hashem" should not be interpreted in the simple sense, (for Yaakov did not build such a structure). Rather, he meant it would be a place to worship Hashem.