What is this Pasuk coming to add?
Rashi: It is adding a Lo Sa'aseh on whoever does not sustain the Levi. 1
See Sifsei Chachamim. Refer also to 12:19:151:1 and note.
What are the implications of "Al Admasecha"?
Rashi: It implies that the obligation of feeding the Levi is confined to Eretz Yisrael, but in Galus a Levi is no different than other Aniyim. 1
See Torah Temimah, note 71.
Why does the Torah insert the words "Pen Ta'azov es ha'Levi Kol Yamecha"?
Sifri: To include even during the years of Sh'mitah and Yovel. 1
Rashi (in Pasuk 18) writes that one who does not have Ma'aser must sustain the Levi from his own food. But later, in 14:27, he writes that he only transgresses "Lo Sa'azov" if he does not give him Ma'aser?
Mizrachi (in 14:27): Here the Torah is discussing eating (sustaining the Levi); there it is discussing Ma'aser.
Gur Aryeh (in 14:27): There, the Torah writes "Ki Ein lo Nachalah." Ma'aser is his inheritance. If you gave him Ma'aser, he has his inheritance (so that Pasuk does not obligate you).