RULE 1 • LONG OO (ōō /uː/) AS THE STANDARD
Most words spelled with oo take the tense Long oo /uː/ sound: food, mood, school, root, boot, toot, shoot, goose, noose, loose, choose, poor.
RULE 2 • SHORT OO (ŏŏ /ʊ/) PATTERNS & EXCEPTIONS
Words in -ook are reliably short (cook, book). Notice the contrast: boot, shoot take long ōō, but foot takes short ŏŏ! Food and mood take long ōō (unlike wood/good).
RULE 3 • CONSONANT DIGRAPHS & SIBILANTS
In school, ch makes hard /k/ (Greek root). In goose, noose, loose, -se makes voiceless /s/; in choose, -se makes voiced /z/.