(Note: I would like to learn Russian, but I have not started yet, so unfortunately I will be making this post entirely in English. I don't mind what language you use for answers.)
It seems that the letter "и" does not generally represent an iotated vowel in Russian at the start of a word. It is pronounced /i/ with no glide ( unlike for example "я" /ja/ or "ю" /ju/).
But I would guess that it is not difficult for a Russian speaker to pronounce /ji/ (sounds like "йи") and to distinguish the sound of this from /i/. I would like to know if I am right about this, and if so, what are some examples of Russian words where /ji/ does occur? And does it occur at the start of any words that you know of?
So far, my research has turned up inconsistent results that I am not sure I can trust.
An old book from 1916, The Russians and Their Language, by Nadine Jarintzov, says:
Only the и, amongst the light vowels, makes a slight exception: there are only three words beginning with it in which it has the y-sound to start it with; these are the genitive, dative and ablative cases of the Russian word for 'they' = они́: ихъ[*] (of them), имъ[*] (to them), and и́ми (by them), yih, yim, yimi. In all the rest of the Russian words beginning with an и the latter (against the nature of other light vowels) has no y-part in it: иго́лка (needle) igolka, и́ва (willow) iva, идти́ (to go) idt’i; etc. (p xvii-xviii)
(*note: I understand that the word-final hard sign "ъ" is not used in the present-day spelling of these words)
The Wiktionary entries on их, им and и́ми describe the pronunciations starting with /ji/ instead of /i/ as "dated or regional".
A little later on, Jarintzov mentions the existence of /ji/ in words like се́мьи. The Wikipedia article "Soft sign" also seems to say that ьи is pronounced as /ji/ in Russian. But since it is an unstressed syllable, I would guess the phonetic pronunciation is actually [jɪ], with vowel reduction causing the vowel /i/ to be less close than it would be in a stressed syllable.
In a Reddit discussion I found, native speakers say not to pronounce /ji/ in им, but a commenter also said that some word-intial unstressed syllables might sound like they start with "йи" because of vowel reduction. The example was
"единорог" is pronounced "йидинарок"
The Wikipedia article "Russian phonology" says a number of things about the pronunciation of vowels in unstressed syllables. Some of them seem confusing or dubious. The parts that I think seem relevant:
When a soft consonant or /j/ precedes, both /o/ and /a/ merge with /i/ and are pronounced as [ɪ]. Example: язы́к About this sound [jɪˈzɨk] 'tongue'). /o/ is written as ⟨e⟩ in these positions.
[...]
The pronunciation of unstressed /e ~ i/ is [ɪ] after soft consonants and /j/, and word-initially (эта́п [ɪˈtap] ('stage')
[...]
There are a number of exceptions to the above vowel-reduction rules:
Vowels may not merge in foreign borrowings, particularly with unusual or recently borrowed words such as ра́дио, [ˈradʲɪ.o] 'radio'. In such words, unstressed /a/ may be pronounced as [ɐ], regardless of context; unstressed /e/ does not merge with /i/ in initial position or after vowels, so word pairs like эмигра́нт and иммигра́нт, or эмити́ровать and имити́ровать, differ in pronunciation.[citation needed]
This is all a bit complicated, so I wanted to make sure I understand.
Here is my summary of what I think I have gotten from all of this:
In word-initial stressed syllables, un-iotated /ˈi/ occurs. Iotated /ˈji/ doesn't occur at the start of any (?) word in standard Russian.
In word-initial unstressed syllables,
[jɪ] (which is equated with /ji/; that is, it sounds like it is pronounced as "йи") occurs at the start of words that are written with iotated vowels like я (from Wikipedia: язы́к) or е (from Reddit: единорог), except sometimes (in "foreign borrowings" in particular?) it seems these may be pronounced without vowel reduction as /ja/ and /je/ respectively (according to an uncited sentence in the Wikipedia article). Update. according to a document I found, linked below, these words may in fact be heard pronounced without /j/, at least "in casual speech". I don't know how common or accepted this is, however.
[ɪ] occurs in unstressed syllables at the start of words that are written with un-iotated front vowels, such as э (from Wikipedia: эта́п) or I assume и (as in игле́, which Wiktionary says is pronounced [ɪˈɡlʲe])
In non-word-initial syllables:
in unstressed syllables, /ji/ (I understand that according to the rules of vowel reduction, this would technically be pronounced [jɪ]) occurs after consonants in words written with ьи, and perhaps after vowels in words written with и or е (the existence of a j-like glide before /i/ after vowels seems dubious, or optional at best).
Perhaps /ji/ can occur in stressed syllables as well? I would appreciate some examples if so. I found a web page that says
Nouns ending with -ий, -ия, -ие (not to be confused with substantivated adjectives) are written with -ии instead of -ие in Prepositional (as this ending is never stressed, there is no difference in pronunciation): тече́ние - в ни́жнем тече́нии реки́ "streaming - in lower streaming of a river". But if words в течение and в продолжение are representing compound preposition meaning "while, during the time of", they are written with -е: в тече́ние ча́са "in a time of an hour". For nouns ending in -ья, -ье, or -ьё, using -ьи in the Prepositional (where endings of some of them are stressed) is usually erroneous, but in poetic speech it may be acceptable (as we replace -ии with -ьи for metric or rhyming purposes): Весь день она́ лежа́ла в забытьи́ (F. Tyutchev).
which confuses me a bit. Does this mean stressed "ьи́" does not occur in any words in non-poetic speech, or are these remarks only applicable to words ending in ьи́?
Update. I found a document, "Russian intervocalic deletion in Derivational Optimality Theory", by Janina Mołczanow (2012), that says that "in casual speech" unstressed syllables starting with [jɪ] may even lose the glide and be pronounced with [ɪ] or after a hard consonant, [ɨ]. The examples given are
"jevropa [i] ‘Europe’ > golos jevropy [s ɨ] ‘the voice of Europe’
jelovyj [i] ‘firry’ – > v jelovom lesu [v ɨ] ‘in the fir forest’
ježa [i] ‘hedgehog’ – > nos ježa [s ɨ] ‘hedgehog’s nose’
(p. 289)
That is, it says that words like Евро́па, ело́вый and ежа́ can be pronounced with the sound "и" (not "йи") at the start, and го́лос Евро́пы can be pronounced with "сы" in the middle, в ело́вом ле́су can be pronounced with "вы" at the start, and нос ежа́ can be pronounced with "сы" in the middle. Russian speakers, does that sound possible to you/are you familar with this type of pronunciation? If so, does it sound OK, or would you definitely recommend that a learner avoid it?
unstressed O -> A, the rest are pretty much provisional, they only simplify pronunciation or are rather a result thereof, and certainly not a part of the school's Russian language curriculum as far as i remember