You may not eat within your gates the tithe of your grain or of your wine or of your oil, or the firstborn of your herd or of your flock, or any of your vows that you vow, or your freewill-offerings, or the contribution of your hand.
English: “You may not” — It was taught: R’ Yehoshua ben Korchah says: [It means] I am able but I am not permitted. Similarly you find (Yehoshua 15:63) “and the Yevusi, the inhabitants of Jerusalem, they could not dispossess” — they were able, but they were not permitted.
On the daf / in context: The language “you may not” is understood as a prohibition of permission rather than a statement of physical inability.
Type of Learning
Derash on the verse (Sifrei) — the force of “lo tuchal.”
What Is Learned
“You may not” means you are capable but forbidden; it is a prohibition, not a description of inability.