One of the stranger letters of the English Alphabet. It spends its life as an orphan and a wanderer, finding work where it can.
C can be used to make the 'K' sound (voiceless velar stop), in words like 'cat', 'music' and 'account'. If it appears at the end of the word, or if it is at the beginning of the word and followed by an 'A', 'L', 'N', 'O' 'R', or 'U', it makes the 'K' sound. In names starting with Mc or Mac it also makes this sound. There are many more rules like this for various situations, such as when 'c' or 'cc' appear in the middle of a word, but if you are sufficiantly versed in English to read this node, you know them already.
C can be used to make the 'S' sound (voiceless alveolar fricative), in words like 'city' and 'accelerate'. If it appears at the beginning of a word followed by an 'E' (except in 'Celtic'), 'I', and 'Y' (except in Welsh-related words, 'cymric', 'cymrite') it makes the 'S' sound.
C can be used in the consonant cluster 'CH' (voiceless palato-alveolar affricated stop). This is the one use in which C can't be replaced. No 'C', no 'Ch'. Words like 'chill' and 'French' depend on it. (Beware 'school'; the 'H' is a non-letter in this word, having the 'C' a make 'K' sound). A few 'Cz' words also make this sound, such as 'Czechoslovakian'.
C is used next to 'K' to indicate that it follows a short vowel (beck, duck). Usually if 'K' is the last consonant sound in a word, it will either be proceeded by 'C' (to indicate a short vowel) or followed by 'E' (to indicate a long vowel). ('back', 'bake'; 'lick', 'like'; 'block', 'bloke').
I should add a link to I before E, except after C, when making the long 'A' or 'E' sound. Details and exceptions can be found at that node.
'C' can be used in for the 'Sh' sound (voiced palato-alveolar fricative, or sometimes voiceless palato-alveolar fricative). This is primarily in words of French origin ('chef', 'chandelier', 'Chicago', 'Michigan', and 'cache'). Likewise, when the 'sh' sound appears in the middle of a word, C can be used followed with 'io' (e.g. 'coercion', 'suspicion', 'atrocious').
And finally, when appearing between a 'S' and an 'E' or an 'I', the 'C' is almost always silent (e.g. 'scene', 'scent'; 'science', 'scimitar'). In those rare cases where it appears at the beginning of a word followed directly by a 'T' (e.g. 'ctene', 'ctenoid', 'ctetology'), a 'N' (e.g. 'cnemis', 'cnidaria', 'cnidoblast'), and in most 'Cz' words ('czar', and related words), the C is silent.
If you have any suggestions for other rules, examples, or etc., let me know.