American English Phonetics

12.5 The Palatal Approximant /j/; Yod-Dropping

In many contexts, historic /juː/ became /uː/ in most varieties of English, a change known as yod-dropping.

Since you may be familiar with British varieties of English, it should be pointed out GA /juː/ does not occur after alveolars, so that tune, dune, suit, assume, presume, lunar, new, student have no /j/.

Following other consonants, /j/ remains, as in British English: [pjuːni,, bjuːti, mjuːz, vjuː, (etc.)] for puny, beauty, muse, view, (/ɛtˈsɛtərə/).

Yod-dropping did not affect syllables after strong syllables, as illustrated by /ˈmɛnjuː:/ menu and /ˈvæljuː/ value.

After /t, d/ in weak syllables, /j/ tends to be assimilated, as in /ˈsætjəˈreɪʃn̩/ saturation, situation, education, which have /t͡ʃə/ and /d͡ʒə/, respectively. The same applies to initial /j/ in the weak form of the pronoun you, as in don’t you (or, informally, doncha) [ˈdoʊnt͡ʃə] and did you [ˈdɪd͡ʒə].