German has a relatively strict mapping between sounds and letters. You can find this mapping-rules in this document, in § 1: Regeln und Wörterverzeichnis
Some of these rules are:
- The short vowels [ʊ] and [u] and the long vowel [u:] are written with the letter »u«
([u:] sometimes also as »uh«)
- The short vowels [ʏ] and [y] and the long vowel [y:] are written with the letter »ü«
([y:] sometimes also as »üh«)
- The short vowels [œ] and [ø] and the long vowel [ø:] are written with the letter »ö«
- Mund = [mʊnt]
- zuvor = [ʦuˈfoːɐ̯]
- gut = [ɡuːt]
- Kuh = [kuː]
- hübsch = [hʏpʃ]
- Büro = [byˈʀoː]
- trüb = [tʀyːp]
- kühl = [kyːl]
- löschen = [ˈlœʃn̩]
- Ökonomie = [ˌøkonoˈmiː]
- schön = [ʃøːn]
Conclusions of this rules are (for German words only):
- The letter u is never pronounced as [ʏ], [y], [y:], [œ], [ø] or [ø:].
- The letter ü is never pronounced as [ʊ], [u], [u:], [œ], [ø] or [ø:].
- The letter ö is never pronounced as [ʊ], [u], [u:], [ʏ], [y] or [y:].
Note, that this rules apply only for German words. There are exceptions for foreign words like »Buffet« = [bʏˈfeː] which are pronounced very similar to the pronunciation in their original language (French in case of Buffet).
Jung is a German word, which means, that it obeys these rules. So, the pronunciation of jung and jünger are:
- jung = [jʊŋ]
- jünger = [ˈjʏŋɐ]
Note, that in German there also exist some minimal pairs for [ʊ]-[ʏ]
- Mutter - Mütter (mother, mothers)
[ˈmʊtɐ] - [ˈmʏtɐ]
- drucken - drücken (print, press)
[ˈdʀʊkn̩] - [ˈdʀʏkn̩]
I have heart some northern dialects, where the letter »i« sometimes is pronounced as [ʏ] instead of [ɪ] (»Fische« = [fʏʃə] instead of [fɪʃə]), but I'm not aware of any German dialect, where »u« is pronounced like »ü« or vice versa. And I also don't know any German dialect, where »u« or »ü« would sound like »ö«.
In German there also is no synaeresis that would eliminate the consonant [j] before any vowel.