Finite is pronounced ['faɪnaɪt] while infinite is pronounced [ˈɪnfɪnɪt]. So why is the vowel in the first syllable of 'finite' different from the vowel in the second syllable of 'infinite'?
OK, it's not because of Trisyllabic Laxing, I made an erroneous assumption which I deeply regret.
The answer is actually simple. As Luigi Burzio explains in Principles of English stress, the reason boils down to English stress patterns.
The diphthong [aɪ] (as in bite) almost never occurs in unstressed syllable. The diphthong [aɪ] has a systematic relationship with the short vowel [ɪ]. This relationship is also reflected in Trisyllabic Laxing; divine-divinity, derive-derivative and in drive-driven etc.
From this relationship, we can infer a general rule of thumb that [aɪ] will only occur in stressed syllables and when that syllable gets unstressed, [aɪ] will shorten to [ɪ].
Now when you prepend the prefix in- to finite, the primary stress moves to the prefix in- because it's a stress-bearing affix.
- In + f[aɪ]nite → inf[ɪ]nite
- In + p[əʊ]tent → imp[ə]tent
- In + m[aɪ]grant → imm[ɪ]grant
- In + f[eɪ]mous → inf[ə]mous
Another example would be cycle-bicycle:
- Bi + c[aɪ]cle → bic[ɪ]cle
There are exceptions, however. Luigi Burzio has explained all the rules and exceptions thoroughly in his book. One of the many exceptions is the prefix un- which doesn't take primary stress, for instance, unab[eɪ]ted.
Latinate words in English tend to be stressed on the penultimate (second last) syllable unless that syllable is short, in which case the primary stress falls on the antepenult (third last syllable) but not on preantepenult (fourth last) as far as I know. That's why impossible, implausible, incredible etc., are stressed on the antepenult.
I think clicking on the website below to listen to the pronunciation, you may tell the difference between them clearer.
Love - verb
Laugh - verb
You may refer to Oxford Dictionary to find more.
Best Answer
In the International Phonetic Alphabet (IPA), finite is /faɪnaɪt/ and infinite is /ɪnfənɪt/.
In case you are unfamiliar with the IPA, "finite" rhymes with "skylight." Both of the vowel sounds are the "long i," as you put it.
You might hear "infinite" pronounced in two slightly different ways. The first way rhymes closely with "run it" (in-fuh-nit) and the second way rhymes more closely with "win it" (in-fin-it). The stress in either pronunciation will fall on the first syllable.
If you are (or are willing to become) familiar with the IPA and its pronunciations, you can find a helpful tool here that transcribes most English words into their IPA form.