What is the overlap between "Y" and "I"?

Solution 1:

In fact, aside from the first two consonantal uses of "Y" already mentioned in simchona's answer, all of the differences I can find go the other way: places where "I" cannot be replaced with "Y" without a change in pronunciation. This is technically the opposite direction from what the question asks for, but I hope this answer is of some use anyway.

1. First, a very minor difference, for which the only example I can find is a name: when transliterating Ancient Greek, the digraph ei can be used to represent the diphthong ɛι, which generally corresponds to a single vowel sound or diphthong in English (most commonly the vowel of PRICE, as in eidolon, eidetic, deictic, Deimos). The letter sequence ey cannot be used for this, and it has a different pronunciation (representing two vowels in hiatus, those of FLEECE and KIT) in the one example of it I can find, the name Ceyx. However, the letter sequence ei can also represent FLEECE-KIT in hiatus rather than a diphthong, as in the word nereid (in these cases, it often corresponds to the Greek sequence ηι, which was not a diphthong). So this is a case where (E)I makes a sound which cannot be represented by (E)Y.

2. Another difference, this one not related to Greek: the digraph ie is fairly commonly used in English to represent the vowel sound of FLEECE (in words ending in -ield such as field, words ending in -ief like grief, and words like niece, piece, and shriek) and the analogous trigraph ier often represents the vowel of NEAR (e.g. fierce, bier). As far as I can tell, ye and yer are never used this way in modern spelling (we can't write fyeld, pyece, fyerce).

3. A third difference: in general, only "I" can be used in modern English as a "silent" letter following a soft "G," "C," "S" or "T." For example, in the words religion and legion, the "I" can't really be replaced with "Y": religyon and legyon don't look like they would be pronounced correctly. This also applies to words like suspicion, session, dictionary.

   In comparison, here are some words that use "Y" in this context:

   • halcyon and the name Procyon are pronounced HALcy-un and PROcy-on instead of ending with -shon or -shun
   • dasyure is pronounced DASSy-yure instead of DASH-ure or DAZH-ure
   • dictyosome and amphictyony are pronounced DICty-osome and amFICty-unny rather than DIC-shosome and amFIC-shunny.
     

   
   You can see that the "Y" is generally pronounced separately, not used to soften the previous letter in the way that "I" is. However, I did find an interesting passage in John Walker's Critical Pronouncing Dictionary that says that, at least in his time, names ending in -cyon, such as Sicyon and Cercyon, were pronounced with a "sh" sound (IPA /ʃ/), as "Sishean" and "Sershean." According to Dictionary.com, in modern times "Sicyon" is pronounced with either /ʃ/ or /s/ depending on the speaker.

As for the name, calling this letter by the equivalent of "Greek I" is simply a shared feature of many Romance languages and other languages that were influenced by them. There's no special detail about how they use the letter that explains why this name wasn't taken into English. In fact, Spanish and French use "y" in mostly the same ways as English:

• for a consonantal value (/ʝ/ in Spanish, /j/ in French), which generally occurs at the start of words or between vowels and which normally is not represented with "i" in these contexts (in both of these languages, "i" is the normal way to represent the sound [j] after a consonant letter)
• as a less common alternate to the letter "i" representing the same vowel sound /i/
• as an alternate to the letter "i" in digraphs such as ay and oy.

Solution 2:

According to the Oxford English Dictionary, while there are some cases in which I and Y can be used to represent the same sound, it is not always the case. They write that there are three situations in which a Y is used:

1. About the middle of the 13th century y began to be used to represent the voiced palatal spirant /j/ , taking the place of the character ȝ (called yogh n., q.v.) in one of its values. -- This use is retained in words like "year", "yet" and "young"

2. Another value of y arises from the assimilation of y and þ, the runic thorn (see th n.), which had become indistinguishable from each other in some MSS. of the early 14th century (e.g. the Cotton MS. of Cursor Mundi). After 1400 þ fell more and more out of use, and in some scripts was represented only by the y-form in the compendia ye, yt or yat, yei, ym, yu = the, that, they, them, thou, and the like, many of which continued to be extensively employed in manuscript in the 17th and 18th centuries. -- This use has largely fallen out of common practice

3. In later (West Saxon) Old English, y was written alternatively for i, e.g. as representing older ie, as in cyle, ongytan, yld, for ciele, ongietan, ield

In its third use, I and Y were interchangeable into Middle English, but after that orthographic rules were put into place, and Y was only used in the following cases:

(i) for final i-sounds, as in fly, family, daily, destroy (formerly spelt also flie and flye, familie and familye, etc.), only alien words being spelt with final i;

(ii) in Greek words, representing υ, as in hymn;

(iii) before i, in inflexional forms of verbs ending in y or ie, as flying, lying, tying, not fliing, etc.;

(iv) in the plural of nouns ending in y preceded by another written vowel, as boy boys, ray rays, alley alleys, money moneys (but monies is still common, and vallies, monkies, etc. were equally so until recently).

So in some instances, I and Y could be interchanged because they can represent the same vowel sound. However, there are now rules about when Y should be used to present this same vowel sound at all.

Solution 3:

No, they're not supposed to be identical.

'i' is pronounced in many ways in modern English. 'y' is also pronounced in many ways, and these two sets overlap, but are not identical.

The major differences are that - 'y' can be used syllable initial to represent the semi-vowel yod (as in 'yes') for which by English orthography rules, you cannot use 'i'. - 'i' can be used for a short close front-mid vowel as in 'bit' where you cannot use 'y'.

In general 'i' is short and 'y' is long. But since English orthography is chaotic, there is lots of variation in pronunciation, such as the ones you point out.