לא תוכל. בָּא הַכָּתוּב לִתֵּן לֹא תַעֲשֶׂה עַל הַדָּבָר: לא תוכל THOU MAYEST NOT [EAT WITHIN THY GATES THE TITHE OF THY CORN etc.] — Scripture thereby intends to attach to this matter (the eating of מעשר בכור etc. outside the walls of Jerusalem) a negative command also (the positive command being contained in v. 11).
לא תוכל. רַבִּי יְהוֹשֻׁעַ בֶּן קָרְחָה אוֹמֵר יָכוֹל אַתָּה, אֲבָל אֵינְךָ רַשַּׁאי, כַּיּוֹצֵא בוֹ (יהושע ט"ו) "וְאֶת הַיְבוּסִי יוֹשְׁבֵי יְרוּשָׁלַיִם לֹא יָכְלוּ בְנֵי יִשְׂרָאֵל (יְהוּדָה) לְהוֹרִישָׁם", יְכוֹלִים הָיוּ, אֶלָּא שֶׁאֵינָן רַשָּׁאִין לְפִי שֶׁכָּרַת לָהֶם אַבְרָהָם בְּרִית כְּשֶׁלָּקַח מֵהֶם מְעָרַת הַמַּכְפֵּלָה; וְלֹא יְבוּסִים הָיוּ, אֶלָּא חִתִּיִּים הָיוּ, אֶלָּא עַל שֵׁם הָעִיר שֶׁשְּׁמָהּ יְבוּס, כָּךְ מְפֹרָשׁ בְּפִרְקֵי דְּרַבִּי אֱלִיעֶזֶר (פ' ל"ו), וְהוּא שֶׁנֶּאֱמַר "כִּי אִם הֱסִירְךָ הָעִוְרִים וְהַפִּסְחִים" (שמואל ב ה') – צְלָמִים שֶׁכָּתְבוּ עֲלֵיהֶם אֶת הַשְּׁבוּעָה: לא תוכל THOU MAYEST NOT (lit., thou canst not) [EAT etc.] — Rabbi Joshua the son of Korcha said: You can, but you are not allowed to (i.e. לא תוכל does not mean: you are unable for some reason or another to do so, but it expresses the inability to do it because of a legal restriction). A similar case we have in (Joshua 15:63) “As to the Jebusites, the inhabitants of Jerusalem, the children of Judah could not drive them out” — they could have done so, but they were not allowed to (Sifrei Devarim 72:1); because Abraham had made a covenant with them when he bought from them the cave of Machpelah that they would be spared at the conquest of the Land. — As a matter fact they were not Jebusites but Hittites (the people mentioned in Genesis 23 at the purchase of the cave), but they were called Jebusites after the city the name of which was Jebus. Thus is it explained in the Pirkei DeRabbi Eliezer 36. And this is the meaning of what is stated, that when David was about to drive out the Jebusites, they said to him (II Samuel 5:6) “Except thou take away the blind and the lame [thou shalt not come in hither]” — by “the blind and the lame” they meant the idols (that stood at the gates) upon which they had written the oath which Abraham had taken.
ובכרת בקרך. אַזְהָרָה לַכֹּהֲנִים: ובכרת בקרך [THOU MAYEST NOT EAT WITHIN THY GATES THE TITHE OF THY CORN …] AND THE FIRSTLINGS OF THY HERD — This, in contradistinction to מעשר דגנך וכו׳, is of course a prohibition addressed to the priests (since the Israelites were not at all permitted to eat firstlings; cf. Zevachim 5:1: הבכור נאכל לכהנים והמעשר לכל אדם).
ותרומת ידך. אֵלּוּ הַבִּכּוּרִים (ספרי; מכות י"ז): ותרומת ידך AND THE HEAVE-OFFERINGS OF THY HAND — This refers to the first-fruits (cf. Deuteronomy 26:4) (Sifrei Devarim 72:9; Makkot 17a).