Open wide like short a, but now pull your tongue BACK and let your lips go round — /o/, as in 'hot'.
Check yourself: Mirror: wide open, but the lips are round, not smiling. That is what separates it from short a.
Watch for: /æ/ — the jaw is the same, the tongue and lips are not.
◆ marks a word that does NOT follow today's pattern — Blumenfeld boxes these on the page. Teach it by heart; do not let the class sound it out and lose faith in the rule.
Say the word, have them repeat it, count the sounds on fingers, THEN write. The count is what stops a child writing the first letter and guessing the rest.