/i:/ is the vowel that we find in the word FLEECE. I put that word in capitals because that is how that vowel is often referred to by linguists: the fleece vowel - or FLEECE for short. (This is not random, the word was specifically chosen for a number of specific reasons.) It is the vowel sound at the end of the word guarantee. In transcriptions of British English it has a colon [ : ] in the symbol to describe the length.
/ɪ/ is the vowel in the word KIT. It is known as the kit vowel - or KIT for short. It is the vowel we find in prefixes and suffixes, the bits we stick onto the beginnings and ends of words. So, for example it is the vowel we hear in --ing verb endings.
The vowel represented by /i/ at the ends of words in dictionaries is usually referred to as the happy vowel - HAPPY. This vowel may sound like either FLEECE or KIT, but is always short in duration.
If you say the < y > sound that we find in the word yes, and then say the < e> we find in the word end, the kit vowel is somewhere between the two sounds. This is the first vowel in the word infinitely. This word would sound very odd to a native speaker if it was said with a fleece vowel, /i:/! It would sound like a made-up word: eenfinitely.
The Original Poster asks if there is a big difference between these vowels. If we are talking about the physical difference between the sounds, the answer is: no. In fact, it is very unusual to have two vowels that are so similar in one language. They are very close together. In most languages these would count as one vowel. However, if we are talking about the meaning, or the effect on a listener, the answer is: yes! There is a big difference. There are very, very, very many words that we can be confused about if you say the wrong vowel. For example, the words peace and piss. Nobody wants to say Piss man!, when they mean Peace man!.
If you want to type IPA script, this website is very useful
Hope this helps,
Peace!
NOTE: This is off the top of my head; there may be aspects of this I've overlooked, so I would welcome any correction anybody wants to supply.
You're missing a simpler way of understanding this, because you're working off letters (which should properly be enclosed in ‹› rather than []) instead of sounds (specifically, phonemes, which should properly be enclosed in // rather than []).
Here are the rules:
Your two suffixes are what linguists call archiphonemes, which are sound-types "realized" as different phonemes in different sound contexts. We may designate them as /S/ and /T/, and categorize them as
/S/ - (dental/alveolar) sibilant, realized as either voiceless /s/ or voiced /z/
/T/ - dental stop, realized as either voiceless /t/ or voiced /d/
How the archiphoneme is realized depends on the sound which ends the base word and therefore immediately precedes the suffix:
- If that sound is voiced, the suffix takes its voiced realization, and
- if that sound is voiceless, the suffix takes its voiceless realization, EXCEPT THAT
- if that sound is of the same category as the suffix, an unstressed vowel is inserted between the base and the suffix so you can hear the two sounds as distinct. Since all vowels are by definition voiced, a voiced sound now precedes the suffix, and the suffix takes its voiced realization. The vowel itself may be realized as anything in the approximate range [ɛ] – [ɪ], usually 'reduced' to [ə] – [ᵻ].
As far as I know, these rules are invariant in Standard English; but when both a plural and a possessive /S/ are applied (e.g., the Joneses' house) one may be suppressed in speech, I believe many Scots dialects realize /D/ fairly consistently as /ɪt/, and there may be other variations I've overlooked or I'm not familiar with.
Similar rules govern prefixes; but since nowadays most prefixes are of Latin or Greek origin, the rules are mostly derived from practices in those languages.
Best Answer
The letter G is called /dʒi:/ in both British and American English. It rhymes with see.
The letter J is called /dʒeɪ/ in in both British and American English. It rhymes with say.