The vowel system is the most complicated part of the pronunciation-spelling system of English, because each vowel letter represents three or four different vowel sounds. The letter “U” has three different sounds, but one thing that is unique about “U” is that it has two Long-vowel sounds.
The basic sounds of the English letter “U” are: Long-U-1, Long-U-2, and Short-u.
The sound of Long-U-1 is the same as the name of the letter “U”. However, this long-vowel sound is a little bit unusual because Long-vowels usually have two parts, but Long-U-1 has three parts (in IPA: /yuw/). Some common words with this sound are: use / music / huge / cute / unite / cure / menu / fuel / human / argue.
The second Long-U sound is almost the same as Long-U-1, except that it has only two parts (in IPA: /uw/). Some words with this sound are: true / flute / blue / June / spruce / tune / rule / tube / duty / include.
Short-u is pronounced in the center (not front, not back) middle (not high, not low) part of the mouth, and the tongue needs to be relaxed. Some common words with this sound are: up / just / but / much / under / us / run / study / number / because.
Any vowel letter can use the schwa sound in unstressed syllables. However, since the schwa sound and Short-u are both made in the same place in the mouth, they end up sounding the same. A linguist would make a distinction between Schwa and Short-u based on stress, so for example, the first letter of the word “under” would be considered a Short-u sound, but in the word “upset” the first letter would be called a Schwa. The good news for learners of English is, you can treat them as the same sound, and your English pronunciation will sound great.
So remember, it is very rare to find the letter “U” pronounced with something other than these three basic sounds. There is a handful of words with a “U” pronounced as Short-oo: sugar / put / push / puss / pull / full / bull / cushion. There are also two words with a very unusual pronunciation, “busy” and “business”. In these two words the “U” has a Short-i sound! Otherwise, when you see the letter “U” in a word, it will almost always have the sound of either Long-U-1, Long-U-2, or Short-u (Schwa).
(NOTE: Words with the vowel pair “OU” are a special tricky category. See: OU – Oh no!)