american english - Why are "creature" and "creative" pronounced differently?

Both the words "creature" and creative" share "creat" in common so they should be pronounced same, right? So why is there a difference?

According to these links "creature" is pronounced as "kree-cher" whereas "creative" is pronounced as "kree-ey-tiv", why "ey" added in next than in the first.

Is there any rule to remember where to use "ey" and where not to?

Answer

This is just a weird facet of modern English pronunciation. I don't know any particularly good explanation or mnemonic. "Creature" and "creative" are actually both pronounced somewhat irregularly/unpredictably, although in different ways (in "creature", the sequence "ea" is pronounced in an unexpected way, and in "creative", the stress is placed in an unexpected place).

Some background material: the digraph "ea" is often used in English to represent a single vowel sound, and its etymological origin in these contexts is usually a single vowel sound or diphthong: e.g. "beak" from French bec, "bean" from Old English bēan (Old English "ea" was a diphthong, not a sequence of two separate vowels), "treat" from Old French tretier, traitier (modern French "traiter").

The spelling and pronunciation of "creature" appears at first glance to fit this pattern, but actually, the "ea" in "creature" doesn't come from a historical monophthong or diphthong. It comes from two vowels in hiatus (the OED says "< Anglo-Norman creatur, Anglo-Norman and Old French criature, Anglo-Norman and Old French, Middle French creature (French créature)").

Regarding the pronunciation, the OED says

The stem vowel appears to have shown some variation in early modern English, but generally to have shown the reflex of Middle English open ē; the different developments of this sound in different regional varieties of English largely account for the variation in the stem vowel which is found subsequently. (There is also very limited evidence for a trisyllabic pronunciation in early modern English, on the same pattern as creation n.) critter n. shows a variant with shortening of the stem vowel after raising to /iː/. The realization of the second syllable varied in Middle English and early modern English according to whether this syllable showed secondary stress; pronunciations of the type /ˈkriːtʃə/ ultimately reflect pronunciations with secondary stress, while pronunciations of the type /ˈkriːtə/ ultimately reflect pronunciations without such secondary stress. Compare forms at nature n., pasture n., etc.

In trisyllabic or polysyllabic words taken from French, stress often seems to move "back" from the final syllable in an alternating fashion. I'll give an example: the word "relative", from French relatif/relative, is thought to have been pronounced with stress on the final syllable in the past (the modern French pronunciation sounds to an English ear like it has final stress). The present-day stress on the first syllable in English can be explained as a strengthening of an originally secondary stress on the first syllable. Alternation of stressed and unstressed syllables is known to be a tendency in English pronunciation: for example, compare the pronunciation of "prohibit", where the first syllable has no secondary stress because it immediately precedes a stressed syllable, to that of "prohibition", which seems to have secondary stress on the first syllable.

Anyway, the alternating-stress explanation is a bit hypothetical, but we can verify that many trisyllables and polysyllables from French are currently pronounced with stress on the third-to-last or "antepenultimate" syllable. (There are classes of regular or predictable exceptions to this stress pattern, like words with a "heavy" consonant cluster after the second-to-last syllable, e.g. "collective" or "deceptive".)

Creative

"Creative" is an unpredictable exception to the general tendency towards antepenultimate stress in words of this type: there is no obvious explanation for its penult stress that I know of (it only has a single consonant after the stressed syllable).

It seems likely that "creative" has a stressed, "long" vowel in the second-to-last syllable in part due to influence from the verb "create". Word-final stress is somewhat regular in certain types of disyllabic verbs. (See this question for notes on the pronunciation of other adjectives ending in -ative: Is there a rule for the position of the accent (stressed sound) in words ending with -ative?)

It may also be related to the use of penult stress in the noun "creator": note that all speakers use this stress pattern for this word, while some other nouns ending in "-ator" like "narrator" and "dictator" may be pronounced with antepenultimate stress.

Creature

According to the alternating-stress/antepenultimate stress principle (which, as mentioned, doesn't always accurately predict the position of stress in modern English), the word "creature" would have been expected to be pronounced something like /ˈkriːətʃər/. However, in a sequence of a stressed vowel immediately followed by unstressed schwa, like [iːə], it's relatively easy for the schwa to be lost. That seems to have happened here. It's also possible that the disyllabic pronunciation was strengthened by the many words spelled with "ea" representing a monophthong, including the now-rhyming "feature" from Old French feture, faiture.