A hard sound, like K, when c comes before a, o, u, and consonants.
coffee, cupcake, school
A soft sound, like S, when c comes before i, e, y.
circle, celery, cyanide, recipe, fascinate
A hiss sound, like sh, when —ci— (or sometimes —ce—) comes before a vowel, inside a word. [Infrequent.]
These words do not have the sh hiss:
Some words that derive from Middle English are pronounced the same as they always were, but the spelling was changed to add a c, similar to French or Latin, which were fashionable languages at the time.
indict, indictment, indictable offence
Other words were imported into English from another language.
yacht [Dutch], Tucson [Papago Indian]
See also, Letter combinations (digraphs), below, for words containing sc.
English words with double consonants are inconsistent, so there is no reliable rule.
Usually, a double C serves to signal that the preceding vowel is short rather than long.
accord, accuse, occupy
Rather than a double C or a double K, a CK is commonly used. The hard C sound is preserved or emphasized, even when, with the following vowel (I, E,or Y), you would expect a soft C sound.
chicken, hockey, sticky wicket
Sometimes a QU is used to preserve the hard C sound.
liquor, chequers (also, checkers [US])
Sometimes a double C is used in a word to express both a hard C and a soft C sound.
Sometimes a single C acts like a double C.
crocodile, decade, second
Letter combinations (digraphs)
c and h together create a distinct sound, a digraph.
cherry, cheque, chi, chutney, church
chemistry, crochet, chenille, Christ, chronology
c and k together create a digraph.
brackets, crickets, flock, quick, rocket
—ct is an uncommon letter combination
s and c together are a digraph that emphasizes the s sound.