Why do the vowels in "finite" change when "in" is added to it [duplicate]

I believe we pronounce infinite the way we do because a dactyl is simply easier to say, in most cases. It trips (quite literally) off the tongue. It seems we're forced into the spondee of finite, even though that is harder to say, because nothing else makes sense. But once that extra syllable has come to the rescue we can get lazy again.

Edit

After giving the matter more thought, I think we pronounce finite as a spondee because both syllables have the "long i" sound (aɪ). I can't think of a single instance of aɪ coming on an unstressed syllable. I think that's mainly true of all the "long" vowels as well, but I will not be surprised if someone can come up with an exception or three.

It's the three syllable rule that messes it up. Logically FI-nite would be in-FI-nite with the prefix, but as it becomes three syllables, the third syllable from the end is stressed and it becomes IN-fi-nite. When you stress the first syllable, you have to change the pronunciation to un-stress the second syllable.

I think Robusto is right that it is related to stress. The stress change causes the unstressed vowel after the prefix to become reduced.

Two stress patterns exist for the prefix in-

You say that "in general, the prefix "in" negates an adjective, but does not change the pronunciation." I would agree with that, but note that normally the prefix in- is also unstressed, even when it is the third syllable from the end : look at the word infertile /ɪnˈfɜrtəl/, which is the negative counterpart to fertile /ˈfɜrtəl/.

So, I think we have to differentiate two versions of the prefix: the more common and productive version in₁- does not take stress and does not change the pronunciation; but there is another, rarer prefix in₂- that does take stress and may cause other changes in pronunciation. This distinction also applies to some other prefixes, by the way, such as re-, de- and pre (compare definite and deform). Obviously, both versions of the prefix have very similar meanings, and in some cases different speakers may use different versions. But I have not heard of anyone saying /ɪnˈfaɪnaɪt/, so I think this particular word only exists with the prefix in₂-.

Here are some more words with in₂-:

• ínfamous
• ímpotent
• ímpious (coëxists with an adjective impíous that has the prefix in₁-)
• ínvalid (n.) (coëxists with an adjective inválid that has the prefix in₁-)
• ínnocent
• ínsolent
• ínfant

Some of these are valid words without the prefix, others are not.

Comparing words with and without the prefix, we can see the kind of sound changes it causes:

• famous /ˈfeɪməs/, infamous /ˈɪnfəməs/
• potent /ˈpoʊtənt/, impotent /ɪmpətənt/
• pious /ˈpaɪəs/, impious /ˈɪmpiəs/

The prefix does not only receive stress, it converts the following syllable into a fully unstressed syllable. In English, vowels in unstressed syllables are reduced.

So that explains the difference in the pronunciation of "fin".

The failure of "silent e" after some unstressed syllables

I'm still not sure exactly why there is a difference in the pronunciation of the final syllables of finite and infinite. I haven't found any other pairs of related words like this that alter the pronunciation of the vowel in the last syllable.

There are a few other words that also show a short unstressed "i" before a silent e like this:

• hypocrite
• requisite
• exquisite
• apocrine

It may be related to having stress on the third-to-last (antepenultimate) syllable. All of these words have this stress pattern, or at least they did originally: nowadays exquisite is often stressed on penultimate.

There are also numerous adjectives and nouns ending in -ate pronounced as /ət/, e.g. associate, delicate, desolate, deliberate, delegate, desperate.

I believe its a matter of stress. IN-fi-nite. FI-nite. It is very awkward to say a word with two stressed syllables followed by one unstressed.

(cross posted with the very useful link to Pronunciation differences between “finite” and “infinite”)

From OED:

Infinite - Etymology: < Latin infīnītus unbounded, unlimited, < in- (in- prefix3) + fīnītus finite adj. and n.; perhaps originally through Old French infinit, -ite (13th cent. in Hatzfeld & Darmesteter), later infini (Oresme, 14th cent.).

In stark contrast to the word “finite” which (i) has only ever been spelled with a final ‘e’, and (ii) entered the language at least 100 years after “infinite”, the word “infinite” entered English in the 1200s and is often found spelled without the “e”. (The final “e” in French words is not pronounced and does not influence the preceding vowel sound.)

Forms: ME–15 infynyt(e, ME–16 infinit,

(OED) This gives an idea as to the early pronunciation.

Thus we have the separate introduction of finite and infinite into the language with “infinite” entering significantly before “finite”, which is the reverse of what one might expect.

In the Latin, the first “i” was short and the second two long /i:/. In French, the first “i” would have been nasal (a unknown sound in English) and the second two long. However, in English, and without the final “e”, all the vowels are short with the final “i”, through lack of emphasis, reducing to a schwa.

There are two meanings to infinite:

1. OED Infinite - /ˈɪnfɪnɪt/ A. adj.

1.a. Having no limit or end (real or assignable); boundless, unlimited, endless; immeasurably great in extent, duration, or other respect.

And

4.b. Of a quantity or magnitude: Having no limit; greater than any assignable quantity or magnitude (opposed to finite). Of a line or surface: Extending indefinitely without limit, and not returning into itself at any finite distance (opposed to closed).

2.

Infinite – /‘inˈfʌɪnʌɪt/ This is occasionally heard and copies the pattern of “non-finite” = /nonˈfʌɪnʌɪt/ and has the meaning of not definite or undefined (Similar to the obsolete meaning of inifinite: †3. Indefinite in nature, meaning, etc.; indeterminate. (OED))

The change in vowel sound is an extension of a change of emphasis used for clarity and to remove ambiguity whilst still conveying the general idea expressed by the root noun.

Similar differences in the vowel sound also are seen in the pronunciation of

Contract – (i) to shorten and (ii) to employ under a contract