What does the Pasuk mean when it writes that "Sarah took Hagar" (which appears superfluous)?
Rashi: It means that she talked Hagar into agreeing 1 to marry Avraham, by telling her how fortunate she would be to cleave to a man as holy as Avraham.
Gur Aryeh: One can "take" an object, but not a person, for a person is defined by his thinking (Da'as), which cannot be "taken" forcefully. Rather, "taking" a person means verbal persuasion, aimed at his reason. (This is reflected in Targum Onkelus, where "taking" a person is consistently translated with the root "Devar.") Also refer to 2:15:0.1:1.
What is the significance of "ten years"?
Rashi: That is the time-period that a man may live with a wife and try to have children, before becoming obligated to marry another woman in order to fulfill that Mitzvah. 1
Gur Aryeh: But perhaps it happened in this case that ten years had passed, and not because of their Halachic significance? If so, the phrase "at the end of ten years" would be superfluous.
Why does the Pasuk specify that ten tears had elapsed "since Avraham settled in Eretz Kena'an"?
Rashi: Because once a person moves to Eretz Yisrael, the years that he lived in Chutz la'Aretz do not count. 1
On what basis did Avraham agree to live with Hagar, who was a Shifchah, and to whom the prohibitions of "Lo Sih'yeh Kedeishah... Lo Yih'yeh Kadeish" (Devarim 23:18) therefore applied?
Targum Yonasan: In fact, Sarah set Hagar free. 1
The question remains however, that the Torah itself constantly refers to her as "Shifchah." (See for example, the next two Pesukim, where Sarah is also referred to as her mistress.) Moreover, the angels who subsequently spoke to her referred to her as a Shifchah! (Also refer to 16:6:1:1, 16:6:2:1.) Also see Ohr ha'Chayim to 16: 6-7.
QUESTIONS ON RASHI
Rashi writes: "Living in Chutz la'Aretz does not count [towards the ten years], because the promise of 'I shall make you a great nation' (12:2) was not said until Avraham arrived in Eretz Yisrael." Ramban asks - But this Halachah applies to any person, not only to Avraham [who received this promise]!
Gur Aryeh: Rashi was troubled with the question that Avraham himself should have counted his years living in Chutz la'Aretz, for he had not yet been commanded to live in Eretz Yisrael. Rashi therefore answers that Avraham had also not yet been promised, "I shall make you a great nation."
Rashi writes: "The promise of, 'I shall make you a great nation,' was not said until Avraham arrived in Eretz Yisrael." How then does the Gemara (Yevamos 64a) derive from here that regarding any person, living in Chutz la'Aretz does not count?
Gur Aryeh: Just as Avraham was able to have children only in Eretz Yisrael, so too every person is most suited to the land for which he was created. We can assume that the sin of living in Chutz la'Aretz would be the cause.
Rashi writes: "Living in Chutz la'Aretz does not count [towards the ten years]." To be more precise, if one ascends to Eretz Yisrael, the years living in Chutz la'Aretz do not count; but if one remains in Chutz la'Aretz they do count, and he must marry another after ten years. Why don't we learn from Avraham, who did not marry another during the many years he was in Chutz la'Aretz?
Gur Aryeh: Avraham himself was unable to have children in Chutz la'Aretz; there was therefore nothing to gain by marrying another. Regarding another person, if he ascends to live in Eretz Yisrael, we may presume that it was the adverse affect of Chutz la'Aretz and we allow him another ten years. If he remains in Chutz la'Aretz, however, he may not absolve himself of the Mitzvah of Peru u'Revu forever on the basis of such a presumption.
Rashi writes: ""Living in Chutz la'Aretz does not count [towards the ten years]." How can we learn this Halachah from Avraham, if he was [then] physically unable to father children (Yevamos 64a)?
Gur Aryeh: If Avraham did not re-marry until now due to his own condition, why does the verse add, "at the end of ten years"? Rather, the purpose of this verse is to teach us about other people. Avraham himself would be healed through prayer.