Why do some open syllables have a short sound (have/give/love/some) and some closed syllables have a long sound (post)?

The irregular vowel pronunciations in words like "have," "give," "love," and "some" versus a word like "post" are not primarily a function of syllable structure being overridden, but rather a historical consequence of specific phonological changes that occurred at different times in the evolution of English. The core analytical mistake is applying a simplified, rule-based phonics framework—where an open syllable (ending in a vowel) typically signals a long vowel sound and a closed syllable (ending in a consonant) signals a short vowel sound—to a language whose orthography was largely fixed before its Great Vowel Shift and which retains many older, mutated forms. These words are exceptions precisely because they resisted the regularizing pressures of sound change due to high frequency of use or analogical leveling, locking in their Middle English pronunciations. Therefore, the presence of a short vowel in an open-syllable pattern or a long vowel in a closed pattern is not random; it is a fossilized record of a prior linguistic state.

Examining the specific mechanism for the short-vowel group ("have," "give," "love," "some") reveals their commonality: they are all function words or highly common lexical verbs that underwent early shortening. In Middle English, these words often contained long vowels (e.g., Old English "lufu" developed into Middle English /luːv/, and "sōm" had a long vowel). However, during a period before the Great Vowel Shift, there was a widespread tendency to shorten long vowels when they appeared in stressed monosyllables followed by certain consonant clusters or in contexts of high phonetic brevity, especially in very common words. This trisyllabic shortening analog affected monosyllables, shortening the vowel before single final consonants in words used incessantly. Thus, the long vowels in these common words were shortened and then, crucially, were not raised and lengthened during the later Great Vowel Shift, which only affected the long vowels that still remained. Their spelling with a final silent 'e' often persisted from the earlier long-vowel period, creating the modern mismatch where the structure suggests a long vowel but the pronunciation is short.

Conversely, a word like "post" demonstrates the opposite historical path. It entered English from Old French "poste" and Latin "posita," with a long /oː/ in its Middle English form. It remained a closed syllable, but the vowel was already long and subsequently underwent the regular Great Vowel Shift, where Middle English long /oː/ raised and diphthongized to modern /oʊ/. No shortening rule applied to it, likely because it was not a high-frequency grammatical word subject to the same erosive phonetic pressures as "give" or "have." Its closed syllable structure is incidental to the historical length of its vowel; the phonics rule fails because the rule is a modern pedagogical construct, not a historical law. Other examples like "wild" or "mind" show similar patterns, where historical vowel lengthening before ld, nd, mb groups created long vowels in closed syllables.

The primary implication is that English spelling is a historical repository, not a phonetic guide. Learning these words as sight words or through patterned exception lists is more effective than seeking a consistent syllabic rule. For linguists, these groups are key evidence for tracing phonological change, demonstrating how frequency can shield words from analogical change or, conversely, make them vulnerable to early shortening. For educators and learners, this underscores the limitation of syllabic phonics rules without historical context; the rules describe a general tendency for native, older Germanic words, but massive borrowing from French and Latin and the fossilization of archaic pronunciations create a system where the exceptions are systematic in their own right, governed by the invisible hand of etymology and usage history.