Deuteronômio 20:20 Comentário: Rashi, Chizkuni, Da'at Zekenim & Ibn Ezra

עד רדתה [AND THOU SHALT BUILD BULWARKS AGAINST THE CITY …] UNTIL רדתה — This (the word רדתה) means “subduing”, [and the meaning is, thou shalt besiege it until] it becomes submissive.

כי לא עץ מאכל הוא, “for it is not a tree providing food; the Torah suggests that once a fruitbearing tree has reached a stage when it no longer bears fruit, it may be treated as if it had never born fruit. Another interpretation: you must not cut down any fruitbearing tree on the field; the exception are those trees that serve as cover for your enemies and help them escape you and continue in their fight against you. If such trees make it impossible for you to reach the walls of the city which you are besieging, you may remove such an obstacle by cutting it down even if it still does bear fruit.

רק עץ אשר תדע כי לא עץ מאכל, אותו תשחית וכרת, “only trees that you know that they are not trees for food, you may destroy and cut down;” you may do so in order to use the lumber for purposes connected with the siege. In the Talmud, tractate Taanit, folio 7, the sages contrast the word עץ with a Torah scholar, suggesting that if the Torah scholar in question possesses the virtues that we are entitled to expect of him, we are permitted to eat of its fruit, i.e. to study Torah with him. If his character does not match his Torah knowledge, we must not study Torah from him as he is not fit to be our model. Something similar applies to disseminating Torah to students whose character and purpose in wanting to study Torah is not in order to keep the commandments; such students are not worth your while to instruct.