Both long U and long OO sounds are spelt with a magic e and the digraphs EW and UE.1 The English long U sound /ju/ (or /ju:/) is actually a 2-sound vowel with a Y-consonant sound followed by a long OO sound /u/ (or /u:/). Often words spelled as a long U are pronounced as a long OO, dropping the Y sound /j/. Deciding whether to pronounce a word with a long U or a long OO is difficult and native speakers often disagree on how some of these words should be pronounced.
However there is a general pattern that we can use to decide between long U or long OO. The long U sound is often used after K, G, B, P and M sounds. These are sounds that are not made with the front part of the tongue.2 For example: skew, cupid (The C has a /k/ sound), argue, butte, pew and mule. Often the long OO sound is used after the T, D, S, N and L sounds.3 For example: toupee, dude, soup, avenue and blue.
Some words use a long U in British English and a long OO in American English.4 These include: dew, dupe, nuisance, stupid, tube, Tuesday, tuna, tune. Why is there such a problem with the long U sound and its spelling? English is spelt the way it was pronounced 400 years ago and the pronunciation has shifted greatly. Probably American English keeps the older long OO pronunciation, where British English has evolved into a long U.
This pattern is a very general guideline that does not always work. There is a lot of variation in these pronunciations. Not only are there differences between British and American English, but also within each variety.5 Some words never follow this pattern, examples include: sudoku, venue, and value. When one is unsure how to pronounce a word, and is unable to check a reference, this pattern may be of help, but it is not a rule.
2. Technically these are the non-alveolar sounds, but may be specifically called velar sounds and bilabial sounds. The velar sounds include: K, G, NG* and the bilabial sounds include: B, P and M. *The NG sound rarely precedes a long U or OO sound in these particular spelling patterns in common words.
3. Technically these sounds are made with the front part of the tongue and are called the alveolar sounds; they include T, D, S, Z, N and L. The Z sound rarely precedes a long U or OO sound in these particular spelling patterns in common words. (The oo spelling pattern in zoo and Zulu is not being covered here because they are not confused with long U.)
4. British English contains many local varieties, some of which use a long OO where most speakers would use a Long U. Some American English speakers will use the more common British pronunciation when others do not.
5. Not only do native speakers not always follow this pattern, we do not always agree on when to follow it and when not to. It is so bad that I had to double check every example used in this article (and the preceding Long U ones) in more than one dictionary and finally settled on the pronunciation provided by the OED (www.oed.com).
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