Grammar myths are commonly held beliefs about language rules that are not actually true in practical usage. For example, many people believe that ending a sentence with a preposition is grammatically incorrect, but in reality, it is perfectly acceptable in modern English. Another common myth is that starting a sentence with a conjunction like "and" or "but" is incorrect, when in fact it is often used for emphasis or stylistic reasons.
Practical usage, on the other hand, refers to the way language is actually used by native speakers in everyday communication. This often means that grammar rules are more flexible than traditionally thought, and that language is constantly evolving. For example, the use of singular "they" as a gender-neutral pronoun is becoming more accepted in practical usage, despite traditional grammar rules stating it as incorrect.
In summary, three key differences between grammar myths and practical usage are the flexibility of grammar rules, the evolving nature of language, and the influence of common usage on what is considered correct in language.