If you haven't already done so, you might want to sign up for Merriam Webster's Word of the Day. You'll receive a new word in your inbox each morning, complete with its pronunciation, definition, example sentence, linguistic origin, and word family quiz. It's fun, free, and educational. Best of all, you won't receive any spam emails, just the word of the day.
Suppose the word of the day is circus. This word contains two c's, yet they are pronounced differently, the first one sounds like an s and the second one sounds like a k. How can a dictionary describe the pronunciation of a word? There are three ways, the first a relatively new feature made possible by the internet.
An online dictionary often has a listen button that you can press to hear a person pronouncing the word. It comes through loud and clear on your computer speakers. You can't beat that.
There is an International Phonetic Alphabet (IPA) that encompasses the sounds of most of the languages of the world. As of 2005, the IPA contains 107 letters, 52 diacritics, and 4 prosodic marks. Most of the letters are not English letters, in the sense of a through z. That's pretty obvious, given that there are 107 of them. This is a rather specialized system used by linguists; it is not appropriate for the general public. As a result, most dictionaries don't use it, or else it is presented as a secondary and/or more precise pronunciation of the word.
The word is "spelled" as it would be pronounced. This system uses primarily English letters, assuming they make their usual, i.e. most common, sounds. Thus circus is pronounced SER-kus. The syllable with the capital letters is stressed. Sometimes the stress is indicated by a vertical bar, ˈser-kus. You can see that the first c is rendered as s, and the second c is rendered as k. Vowels are also transmuted to avoid ambiguity. A long o is spelled oh, thus row, note, and soap are pronounced roh, noht, and sohp. Similarly, plate, rain, and weigh are pronounced playt, rayn, and way. This is the easiest way to describe a word's pronunciation to an English speaker who doesn't happen to have a degree in linguistics.
There is one character from the API that sometimes appears in English respellings. It is the schwa, written ə. It represents the null vowel, the sound of your voice with no tonal modifications. Open your mouth just a bit, relax your tongue and lips, and turn on your voice. The result is not a e i o or u, short or long, though it is close to the short u sound, and is sometimes represented as uh. The article a is often pronounced schwa, as in "have a nice day". An unstressed vowel may also be converted to schwa, or it may lie somewhere in between schwa and the sound of the original vowel. Cadillac is pronounced KA-di-lak, or KA-duh-lak, or (typically) KA-də-lak. When spoken quickly in casual conversation, the i becomes a schwa. Almost any vowel can become a schwa, even y, as in syzygy = SI-zə-jee.
Most consonants in most languages can be voiced or unvoiced. Make the sound of the letter f, and while you are making this sound, turn your voice on. The letter f becomes the letter v. In technical terms, f is the voiceless labiodental fricative, and v is the voiced labiodental fricative. Pause, and verify this voiceless / voiced relationship for f and v, s and z, t and d, p and b, k and g, and soft th and hard th.
The latter causes some consternation, since the exact same letters, th, are voiceless in the word think, and voiced in the word that. How do we distinguish between the two sounds in writing? Often we don't. The letters th are included in a word's pronunciation, and you are suppose to know, from the rules of English or the patterns of other words, whether it is voiced or voiceless. Oddly enough, we're pretty good at guessing the right one. This is the only glaring ambiguity in Marriam Webster's pronunciations.
Aside from the simple articles and pronouns, the they those them this thou etc, most English words that contain th come from the Greek, and are often scientific or technical. Our th is the analog of the Greek letter θ. Words with ph also come from the Greek, wherein ph sounds like f, and is the analog of φ.
Here is another fricative voiced voiceless pair. Make the sound of sh, as in shop, then switch on your voice. The result is a sound that we don't have a consistent letter for, though it is definitely an English sound, as illustrated by the following words.
garage measure beige vision montage treasure azure collage Asian
When writing a word's pronunciation, this unusual fricative is represented by zh. Thus measure is pronounced ME-zher. This makes sense if you think about it. Add your voice to s and get z; add your voice to sh and get zh. All our zh words come from the French.
There's another fricative that you have probably heard, though it is virtually absent from the English language. I can only think of two examples, and one of them is a proper noun. Consider the last consonant in J. S. Bach, or in the interjection yech, used to express disgust. This is sometimes represented by the letter x, or ḵ, thus Bach is pronounced boḵ. To make the sound, move your tongue up towards the roof of your mouth, then blow air through the narrow opening. This is, as you may have guessed, a German phoneme, though German words that enter English almost always transmute this phoneme into something else, usually sh or k. We retain the phoneme in the special case of Bach, because, well, it's the man's name, and we allow the phoneme in yech because we're disgusted, I guess. There are a few others, like sprachgefuhl, which appeared in the final round of the 2015 Scripps National Spelling Bee.
Say the word Bach, and extend the final consonant, then turn on your voice. The result is a perfectly good phoneme, though it is not part of English, or any of the languages that routinely feed into English. You may have heard it from a baby when he is playful and happy. This was captured by a recent Gerber commercial. [listen]
When he babbles, a baby produces every phoneme in every language; he then eliminates the sounds that are not reenforced by the speakers around him, the sounds that are not in his language. As an adult, he can't even make those other sounds any more - not without a lot of practice.
An affricate is a sequence of phonemes that seems like one sound to the native speaker. An English example is ch, as in chest. This is actually t followed by sh. Say the word slowly: t sh e s t. This is the voiceless version - add your voice to ch and get j. Other languages have other affricates. The German z is pronounced ts, as in Zimmer = tsimmer, the word for room. Setswana, spoken in Botswana and in South Africa, includes the ts affricate, and also the tl affricate. When I first heard the latter I thought it was pl, as in plate, but my friend explained that it is tl, a non-English cluster.