Further note - traditional Chinese phonology categorizes syllables by their onset and rime. mian is [mjɛn], notionally m- onset and -jɛn rime. yan is [jɛn], notionally zero onset and -jɛn rime. (And analogously for many syllable series involving a glide.)

One distinction that some Chinese speakers fail to make, though, is between the r- onset and what I would prefer to think of as the j- onset. Thus, for these speakers, rang / yang or rou / you are the same sound.

Pinyin ran uses the standard /a/ vowel, but yan does not. I don't know whether, for speakers who don't distinguish r- from y-, a distinction remains in the vowel of ran/yan syllables.