As a non-native speaker, I read pinyin and want to pronounce 'yi' as /ji:/ and 'wu' as /wu:/.
But sometimes I think I hear just /i:/ and /u:/.
Is that really the case or am I just (not) hearing things? I don't ever hear 'yang' or 'wang' as /jaŋ/ and /waŋ/. Are there (other) contexts where the word initial glide does not occur?
This is probably a better question for linguistics stack exchange; however, I will say that you are not hearing things incorrectly. My background is in linguistics, though not specifically in phonetics and phonology, so don't consider this an expert opinion. However, given some searching, I've found a helpful Wikipedia (Glides section) note on this:
The glides may occur in initial position in a syllable. This occurs with [ɥ] in the syllables written yu, yuan, yue, and yun in pinyin; with [j] in other syllables written with initial y in pinyin (ya, yi, etc.); and with [w] in syllables written with initial w in pinyin (wa, wu, etc.). When a glide is followed by the vowel of which that glide is considered an allophone, the glide may be regarded as epenthetic (automatically inserted), and not as a separate realization of the phoneme. Hence the syllable yi, pronounced [ji], may be analyzed as consisting of the single phoneme /i/, and similarly yin may be analyzed as /in/, yu as /y/, and wu as /u/.1:274ff It is also possible to hear both from the same speaker, even in the same conversation.1:274ff For example, one may hear the number "one" 一; yī as either [jí] or [í].
What this seems to be saying is that the glides [j] and [w] can be inserted in front of certain vowels of which the glide is considered an allophone and that the insertion of such glides depends on the speaker. From my own experience, this very much seems to be the case. I often speak with Taiwanese/Southern Chinese speakers and will almost always hear [u] to represent the pinyin "wu" and [i] to represent the pinyin "yi". For what it's worth, this is also considered "standard" in terms of the sources (1,2) I've found.
I am avoiding / / notation since usually this is used to denote phonemes (something used to distinguish minimal pairs), whereas [ ] is used to denote surface realizations of the phoneme (allophones). By the way, if someone more familiar with phonetics/phonology finds something wrong here, please do correct me.
To sum up, you are not wrong in hearing it this way. Just be mindful that you may hear some variations. If you want to be safe and certain that you will be understood, then pronounce them as [u] and [i]. Though, given some more listening practice, you may start to mimic some other accents. Try out this website to test out words like "五" and ”一“. You can filter by region to try to hear differences. Personally, I hear the glides inserted more often when using the CN filter, and find them less so when using the TW filter.
PS: You might also find it interesting to look at "Bopomofo" or "Zhuyin", the phonetic symbols used in Taiwan as opposed to Pinyin. In the case of "一" and "五”, both use symbols that correspond simply to [i] and [u] respectively.
EDIT: To answer your question "Are there (other) contexts where the word initial glide does not occur?". To my knowledge, this is the only context where [w] and [j] are omitted in pronunciation but still have "w" and "y" written explicitly in Pinyin. Someone can correct me if I'm mistaken here.
"High vowel" / "glide + analogous-high-vowel" are non-contrasting in Standard Chinese, and can be pronounced either way:
Chinese also lacks the contrast between V and GV, or VC and GVC, where V is a high vowel and G is the corresponding glide, such as the pairs in (30).
Duanmu refers to this as G-spreading:
G-Spreading requires a high nuclear vowel to spread to the onset. As a result, there is a lack of contrast between many pairs of forms, such as [sin] vs. [sjin] or [ɕin]. The process is graphically shown in (28), where O is the onset, N the nucleus, and C the coda. As will be discussed in Chapter 4, in a CGVX syllable both C and G are in the onset.
Because of G-Spreading, there is no surface [sin] but only [sjin], which is used by some speakers, and [ɕin], which has further undergone palatalization and is used by most speakers (see Chapter 2).
G-Spreading may also be responsible for the lack of contrast between [win] and [wjin] or [ɥin], or between [ juŋ] and [ jwuŋ] or [ɥuŋ], if it can apply, at least sometimes, when the onset already has G. Assuming that [ɥ] is a combination of [ j] and [w], [ juŋ] can be analysed as in (29).
As far as I can hear, Chinese pronounce "yi", "yu" and "wu" most times as [i], [ü] and [u], except sometimes in combination with a preceding syllable (like in "suoyi"). This is the reason, I think, why Mandarin speakers have problems in distinguishing between the pronunciation of e.g. English "east" and "yeast". Also because of this, foreign students of Mandarin are never, or hardly ever, warned in Chinese textbooks not to pronounce the y- resp. w- preceding -i or -u, which in my eyes is a real omission. I am afraid Chinese just don't hear the difference and therefore won't understand the question of issue, neither will they adjust their textbooks for foreigners. For pronouncing the words "east" and "yeast" correctly they would have to do hard exercise. This topic is basically a problem of Chinese textbook authors rather than foreign learmers.
IMHO, there are two reasons.
In general, Chinese people tend to pronounce every Chinese character in one syllable (每个字听起来像一个音节) with similar length, which will cause the "word initial glide", like 'y' and 'w'.
That makes you find different pronunciations between learning Pinyin and actual chinese characters.
Examples:
"王"
If you separate the pinyin, it goes like /u:/ + /a:ŋ/.
But actually, we pronounce "王" as /wɔŋ/ to make it as one syllable.
(And you never heard the sound /u:a:ŋ/ in China)
"一个"
Pinyin goes as /yi:/ + /gə/.
But the word sounds more like /igə/, as "个" is kind of a short sound.
The other reason is that Pinyin has got four tones, which are represented on vowels rather than the initial sound, but there is NO definition of long vowel.
Among those four tones, first tone sounds more like a long vowel while others not.
In this case, you'll find some characters with same vowels in pinyin, but sounds (quite) different.
By the way, those are also (probably) the reasons that when Chinese start to learn English, they usually pronouce long vowels improperly, as they still speak in the way of Pinyin.
(Disclaimer: Not a native speaker )
This depends on the person's speaking habits. Chinese people have told me you can use either one, most people can't or don't care enough to tell the difference.
Sometimes, the same speaker will alternate their pronunciation with the same word within the same conversation. What I noticed was that the vowel portion (so the part after w or j as you said) is clear. If it's two syllables, then there is "glide" in between, rather than breaking up the word.
I don't think of this too linguistically... I just think of it like Spanish or English. It's all about connecting the words and making it easier on your throat and tongue, rather than stopping every time to say each word. A Napple -- this is how we read "an apple". In Chinese "Yinyue" is easier to say as "injwe" rather than "in-i-ue" or something like that.
The letters w and y are just an orthographic convention.
Syllables starting with u are written as w in place of u (e.g., *uan is written as wan). Standalone u is written as wu.
Syllables starting with i are written as y in place of i (e.g., *ian is written as yan). Standalone i is written as yi.
Syllables starting with ü are written as yu in place of ü (e.g., *üe is written as yue). Standalone ü is written as yu.
Further explanation:
The letter "i" in pinyin should be written as "y" if it is the start of a syllable. This is to avoid ambiguity in syllable boundary when writing words. For example, for "fani" it is not clear whether it's "fa-ni" or "fan-i" (both are possible in Mandarin), but for "fanyi" the only possible combination is "fan-yi".
This is only an orthographic convention. Whether it is written as "y" or "i" does not affect the pronunciation. An English speaker might think "yi" is /ji/ and "i" is /i/, but a native Mandarin speaker does not distinguish between /ji/ and /i/.
The same applies to w/u.
There are other conventions. Read them here: https://en.wikipedia.org/wiki/Pinyin#Orthographic_rules
As a russian native speaker I use Palladiya system
russian has more sounds, and it's easier to read than International Phonetic Alphabet transcription
