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he teaches ill who teaches all
01
тот учит плохо, кто учит всему
used to imply that a teacher who claims to know everything and teaches beyond their expertise may be doing more harm than good
Пример
The headmaster warned new teachers against the mistake of he teaches ill who teaches all, advising them to focus their lessons on core principles and develop mastery of essential concepts rather than trying to impart a broad range of facts.