I am not a native English speaker and I have a question.
Why is the sound of the letter i different in the words dinner and diamond? What is the criteria for the change?
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In this answer, I am using the word vowel to refer to the letters "aeiou" and perhaps "y", and not the sounds which are called vowels by linguists.
The English spelling system is only approximately phonetic. The vowels in particular don't match very well, because English has lots of variance in vowel pronunciation between dialects and also because of the Great Vowel Shift. Native speakers tend to mentally compare an unknown written word to their memorized pronunciations of written words similar to the unknown word when they guess at pronunciations. However, there are rules which can be used to guess at the pronunciation. These rules are mere hints and there will be many exceptions.
Traditionally, each vowel letter is assigned two sounds, a "short" sound and a "long" sound. These names do not correspond to length of the sound, but are best thought of as arbitrary classes. The long sounds are the names of the vowel letters in the alphabet. Unfortunately, vowels can also make sounds besides their short and long sounds. For example, in some words, "i" makes the sound "ee" does in "sheet", for example, in the word "ski". Modern linguists do not use the categories of long and short vowel sounds for English in this way, but the rules I was taught in school for spelling use these categories, which is why I use them here.
When vowels occur in bunches like "ia", often the first vowel makes it's long sound and the second vowel is silent. This is the case in "diamond", "bait", or "speak". However, vowels in bunches in some words will be pronounced in a sequence where each vowel takes on one of the sounds which that vowel can make (long, short, or some other sound). An example of this is the "ia" in "variance". I have never learned a rule for determining the sound of those vowels, or which of the cases apply.
When vowels occur in the pattern VCv where V and v are any two vowels and C is any consonants, the first vowel will often (but not always) make a long sound. The second vowel's sound can not be determined by this rule. This rule is the reason for the sound "i" makes in "bite" and "diner" (a type of restaurant, different from "dinner").
When vowels occur in the pattern VCCv where V and v are any two vowels and CC is one consonant repeated, this is a strong hint that the repeated consonant was added to prevent the first vowel from having a long sound. (Repeated consonants in English have the same sound as a single consonant generally.) Thus, the first vowel in this pattern will usually have a short sound. This is why "i" in "dinner" has its sound.