I have learned that "bush" and "rush" are pronounced differently. Is there any rule to decide between these two pronunciations for similar words?
Learn English – Pronunciation of “bush” versus “rush”
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In all of these ‘gn’ words the ‘g’ is silent—not pronounced. They all came into English from French fairly early and have been thoroughly ‘naturalized’.
Words with ‘gn’ which came into English directly from Latin, such pugnacious, preserve the /g/ sound. But some of these have French-mediated cognates without the /g/—impugn is one such. Signature, although it came into English from French in the 14th century, was at some point reinterpreted as if it came directly from Latin signatus, and is pronounced with the /g/. Your magnolia is even more curious—it derives from the name of the French botanist Magnol, but by way of the Latinized botanical name of the plant (which was conferred upon it by an English admirer), so it pronounces the /g/.
There are some ‘gn’ words from Greek; these preserve the /g/ when it follows a vowel (agnostic, anagnorosis), but not at the beginning of a word (gnostic, gnome).
There are many more recent words from both French and Italian which are so to speak ‘permanently resident aliens’; these retain their native pronunciations (more or less—you might not be very happy with our pronunciation of terms from your own language) with /nj/: cognac, lasagne, carmagnole, bagnio and gnocchi.
Finally there are the words starting with ‘gn’ which descended from Old English. These all retained the /g/ into Middle English down to the period when spelling began to be regularized, but subsequently lost it. ‘gn’ inside OE words lost the /g/ sound much earlier, which is why the letter ‘g’ no longer appears in (for example) rain < OE regn.
ADDED:
And Jim reminds us of gnu. Dictionaries license both /nu:/ and /nju:/; the word is variously said to derive via Dutch gnoe from Khoikoi gnou or Khoikhoi i-ngu repesenting Southern Bushman !nu:.
I couldn't find good references by Googling, and I don't know anything about British english. As I think it through, it is quite complicated! Sorry -- we should really get around to some spelling reform. I hope others can help edit this list if they think of exceptions.
In American English, typically
If there are any prefixes or suffixes causing an s to be in the middle of a word (either because the "s" is part of the prefix or because it is part of the root"), the "s" is always unvoiced /s/, e.g. subsist, substandard, mismatch, mistake, etc.
An s that is written next to an unvoiced consonant is always unvoiced /s/, e.g. lisp, rasp, history, etc.
When the unvoiced consonant of the above rule is [t], then the /t/ is silent if the next syllable is syllabic /n/ or /l/: listen, whistle. (Otherwise it is pronounced. See the comments for a more detailed description of this rule.)
An s before m is always voiced /z/: chasm, prism, plasma. However, the top rule takes precedence, so the s in mismatch is always voiceless /s/. I cannot think of another letter besides [m] where s occurs before or after a voiced consonant word-medially (except by prefixing as in example one), but it would be voiced in such a situation.
An s that is written doubled between vowels is also unvoiced (I guess this is a special case of the above rule): massive, missive, missile, etc. However, if the s would occur in the phonetic stream /sj/ then it assimilates to /ʃ/, e.g. in mission.
An s that is written as one single letter between vowels is usually /z/, e.g. laser, risible. In the same environment as mentioned above /zj/ will assimilate to /ʒ/ e.g. in vision.
Terrible exception to the above: in dessert, the s is voiced to /z/. Many native English speakers misspell dessert for this reason. Note also that the difference between desert and dessert is not voicing, but which syllable gets the accent (it is the first in desert and the second in dessert).
Possess and its derivatives are another exception; the middle "ss" is voiced to /z/. The terminating "ss" is not.
Other miscellaneous exceptions: The -ss- in the American state name Missouri is also exceptionally pronounced /z/. In raspberry, the p is silent and the [s] assimilates to the /b/, so is voiced to /z/.
Best Answer
English does not have a phonetic alphabet. Instead of thinking that the spellings reflect pronunciations, you need to see that words have both spellings and pronunciations, and that they are not always consistent with each other.
One reason for this is that English spelling was mostly standardized during the Middle English period of the language. Since that time, the pronunciations have changed while the spellings have stayed the same.
Another reason is that the Latin alphabet is not ideal for the sounds of English. There are 26 letters, but there are 40+ sounds - 5 or 6 vowel letters but 16-21 vowel sounds, depending on dialect; and 20 or 21 consonant letters but 24 consonant sounds. Since English does not use diacritics, we use digraphs where possible. But it's a mess.
For the most part, English speakers learn the pronunciation first, and then memorize the spelling. This is the best idea for learners too.
So, even though bush and rush are spelled similarly, their vowels are different. For most English dialects, bush has the same vowel as look, and rush has the same vowel as up.