Recently I asserted that the third syllable in 'adjectival' is different from the same syllable in 'adjective'. I was asked why that is, and all I could do was mutter something about the addition of the suffix changing it and something about strong and weak syllables, but I really have no idea why.
As John Lawler notes in his comment:
Because it's a stressed syllable, and the spelling suggests that the stressed vowel is diphthong /ay/ ("long I"), rather than the lax /ɪ/ ("short I") of adjective. The real question is why the last syllable of adjective is not pronounced /tayv/, as the spelling suggests. Just English spelling, alas -- it's still Not Ready For Prime Time.
Of course, the general pronunciation pattern for words ending in -ive is that they are pronounced with /ɪv/. But -al adjectives are sometimes pronounced differently from their bases.
The patterns for the pronunciation of -al adjectives are apparently somewhat complicated and it would be difficult to give a summary of them that is both complete and concise.
However, one generalization that holds for almost all words of this type is that if the second-to-last syllable is stressed, and the vowel in this syllable is only followed by a single consonant (in both spelling and pronunciation, e.g. excluding both cases like "affixal", which is spelled with the single letter "x" but pronounced with the two consonant sounds /ks/, and cases like "colossal", which is pronounced with the single consonant sound /s/ but spelled with the two consonant letters "ss"), then the vowel is not pronounced as one of the "short" vowel sounds /æ ɛ ɪ ɒ ʌ ʊ/.
(Actually, for the purposes of this rule, the digraphs th ch ph usually only count as single consonants, and so do certain consonant clusters such as br cr gr tr dr, but discussing that gets a bit more complicated.)
Dabouis (2016) lists only seven -al adjectives for which exceptional pronunciations with a stressed short vowel followed by a single consonant (or a digraph or consonant cluster equivalent to a single consonant) in the second-to-last syllable are possible (none of them are mandatory, because all of the words have regular alternative pronunciations):
(21) words with /(-)10/ and ˈV̆ in open penults:
ˌartiˈs[æ]nal (main pronunciation /0100/),
ˌpyram[ɪ]dal (main pronunciation /0100/ + variant in /1000/),
canˈt[ɒ]nal (+ variant in [əʊ] + variant in /100/),
eˈp[ɒ]chal (main pronunciation /100/),
inˈt[e]gral (main pronunciation /100/) and
palˈp[e]bral (+ variant in [iː] / main pronunciation /100/).
So the pronunciation ˌadjecˈt[ɪ]val, with a stressed second-to-last syllable containing the "short i" sound /ɪ/ followed by the single consonant /v/, would be irregular.
Now, the rule I mentioned above doesn't by itself rule out pronunciations like aˈdjectival or even ˈadjectival. But Dabouis describes words ending in -ival as constituting a specific (small) sub-class of -al words where stress on the second-to-last syllable is regular:
What is striking here is the great regularity of “micro-paradigms” (Girard, 2007) such as -ival or -oidal (both comprise under 10 entries), even though their stress pattern (penultimate) is “marked” for this structure.
The main thing I'm trying to say in this answer is that the pronunciation of "adjectival", strange as it may seem, does in fact conform to some identifiable patterns about the pronunciation of English words. Trying to explain why these patterns exist is a more difficult problem that I won't attempt to solve.
- Quentin Dabouis, « Is the Adjectival Suffix -al a Strong Suffix? », Anglophonia [Online], 21 | 2016, Online since 01 July 2016, connection on 15 March 2018. URL : http://journals.openedition.org/anglophonia/754 ; DOI : 10.4000/anglophonia.754