Provebs about learning

§"He teaches ill, who teaches all."
The unusual structure of this proverb may make it difficult to understand. It becomes easier if we change the structure to "He who teaches all teaches ill." The word "ill" here means "badly". So it means that the teacher who teaches students everything, does not teach well. A good teacher lets students discover some things for themselves.
• ill (adverb) = badly

§"Better untaught than ill taught."
This proverb drops the verb "to be". But we understand: "It is better not to be taught at all than to be taught badly." It's better not to learn something than to learn it badly. This idea is echoed in Pope's famous line: "A little learning is a dang'rous thing;".
• taught = past participle of verb "teach" (here used in passive voice)
• ill taught = badly taught

§"Soon learnt, soon forgotten."
Something that is easy to learn is easy to forget.

§"You can't tell a book by its cover."
We need to read a book to know if it's good or bad. We cannot know what it's like just by looking at the front or back cover. This proverb is applied to everything, not only books.

I liked all these proverbs but mostly the last