Both phonemes sound practically the same, so it's understandable that there are languages such as Spanish and Italian in which /j/ shares grapheme with /i/ in diphthongs. That is, in these and other languages in which /j/ is present, it does so as an allophone of /i/ in diphthongs. However, other languages with /j/ do have a separate grapheme for it (usually j or y). So how is it possible that there are languages that distinguish between both phonemes and others that do not?
2 Answers
First, it's worth noting that phonemes are entirely theoretical constructs. There's no objective, quantitative measurement someone can make to determine whether something is a phoneme or not; it's entirely at the discretion of the person writing the theory, based on what makes the theory more explanatory and elegant. This also means that phonemes are specific to a single language (and in fact to a single analysis of a single language), not universal.
That said, the main difference between /i/ and /j/ as phonemes is that one is a consonant and the other is a vowel. Phonologically, the definition of a "vowel" is generally that it forms the nucleus of a syllable, and a "consonant" doesn't. This means that there can be (near-)minimal pairs between the two, like in Latin: iambus /i.am.bus/ "iamb" versus jam /jam/ "now". /i/ forms a syllable of its own, and /j/ doesn't.
Does this mean that they're always distinct phonemes? Not at all. In Hittite (according to Kloekhorst's analysis), for example, [j] appears only before vowels, and [i] never does, so it makes sense to call them allophones of a single phoneme. It all comes down to the specific language and the specific analysis you're looking at.
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1This personal/subjective comment might demonstrate mechanisms going on "in people's heads": in Italian, the word uovo has <u> sounding like [w], but it was historically part of the main vowel, coming from Latin ovum. So, as a vowel, it takes l' as its definite article: l'uovo. The article il is used before consonants, and I say il whiskey, even though this word also starts with [w]... I think others say l'whiskey, but before realizing it felt different to others, I had simply never thought about it: being in a loanword apparently makes [w] a consonant to me, unlike native words.– LjLJul 29, 2020 at 14:59
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Right now I can't think of a similar pair with [j], but just like [j] is spelled <i> in Italian, [w] is spelled <u>, and I'd wager most native speakers never consider them "not vowels", so the situation is similar.– LjLJul 29, 2020 at 15:02
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I agree that "First ... phoneme or not" (let's call that A). I also agree that "phonemes are specific to a single language" (let's call that B). However, there is nothing whatsoever to support your assertion that "A therefore B". I find it very strange for you to assert this. Aug 2, 2020 at 0:47
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@Araucaria-Nothereanymore. The idea is that phonemes don't exist independently of a specific analysis; you can measure an
[i], and then say various languages all have that sound, but it doesn't make sense to say English/i/is "the same as" Spanish/i/(what would that even mean?).– Draconis ♦Aug 2, 2020 at 1:48
Are there actually any languages that actually distinguish between them?
It's an issue of semantics whether languages actually distinguish between that, or whether they distinguish between the number of syllables or moræ or some other unit.
I can't think of language that distinguishes between, say, /ia/ and /ja/ without a helping distinction such as that one is one syllable and the other two, or one mora and the other two.
Perhaps someone has a counter example — but is there actually a single language on the planet that distinguishes something like /ia/ from /ja/ that does not place both in a different syllable or mora, and thus can be said to actually be distinguishing syllable or mora-count instead?
But this is also your answer why they are distinguished: many languages develop to have meaningful concepts of syllables or moræ where the number of any in a word is consistent and altering this would change the meaning of a word, such a language would create a minimal pair between, say, hypothetically [ti.a] and [tja], for the latter has less syllables than the former.
[i]versus[j], or/i/versus/j/? (Phones or phonemes?)