he teaches ill who teaches all
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used to imply that a teacher who claims to know everything and teaches beyond their expertise may be doing more harm than good
The origin of the proverb "he teaches ill who teaches all" is attributed to the 16th-century scholar and philosopher, John Lydgate, who was an English poet and monk. The saying is often used to suggest that a teacher who tries to cover everything, especially areas outside their expertise, may not be helping students. Instead of providing valuable knowledge, they might mislead or confuse them. It highlights the importance of knowing one’s limits as an educator and focusing on teaching what one truly understands, rather than attempting to teach every subject or concept.
- 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.