English spelling has more complicated rules than many other spelling systems used by languages written in alphabetic scripts and contains many inconsistencies between spelling and pronunciation, necessitating learning for anyone learning to read or write English.
Throughout the history of the English language, these inconsistencies have gradually increased in number. There are a number of contributing factors. First, gradual changes in pronunciation, such as the Great Vowel Shift, account for many irregularities. Second, relatively recent loan words from other languages generally carry their original spellings, which are often not phonetic in English. Inconsistencies in the Romanization of languages using alphabets derived from the Latin alphabet (e.g., Chinese) has further complicated this problem, for example when pronouncing Chinese place names. Third, some prescriptionists have had partial success in their attempts to normalize the English language, forcing a change in spelling but not in pronunciation.
History of the English spelling system
The regular spelling system of Old English was swept away by the Norman Conquest, and English itself was eclipsed by French for three centuries, eventually emerging with its spelling much influenced by French. English had also borrowed large numbers of words from French, which for reasons of prestige and familiarity kept their French spellings. The spelling of Middle English, such as in the writings of Geoffrey Chaucer, is very irregular and inconsistent, with the same word being spelled differently, sometimes even in the same sentence.
The pronunciation /?/ (normally spelled u) of written o in son, love, come, etc. is due to Norman spelling conventions prohibiting writing of u before v, m, n due to the graphical confusion that would result. (v, u, n were identically written with two minims in Norman handwriting; w was written as two u letters; m was written with three minims, hence mm looked like vun, nvu, uvu, etc.) Similarly, spelling conventions also prohibited final v. Hence the identical spellings of the three different vowel sounds in love, grove and prove are due to ambiguity in the Middle English spelling system, not sound change.
There was also a series of linguistic sound changes towards the end of this period, including the Great Vowel Shift, which resulted in "igh" in "night" changing from a pure vowel followed by a palatal/velar fricative to a diphthong. These changes for the most part did not detract from the rule-governed nature of the spelling system; but in some cases they introduced confusing inconsistencies, like the well-known example of the many pronunciations of "ough" (rough, through, though, trough, plough, etc.). Most of these changes happened before the arrival of printing in England. However, the arrival of the printing press merely froze the current system, rather than providing the impetus for a realignment of spelling with pronunciation. Furthermore, it introduced further inconsistencies, partly because of the use of typesetters trained abroad, particularly in the Low Countries.
By the time dictionaries were introduced in the mid 1600s, the spelling system of English started to stabilize, and by the 1800s, most words had set spellings.
Irregularities in the English spelling system
The English spelling system is one of the most irregular spelling systems in current use. Although French presents a similar degree of difficulty when encoding (writing), English is more difficult when decoding (reading). English has never had any formal regulating authority, like the Spanish Real Academia Española, Italian Accademia della Crusca or the French Académie française, so attempts to regularize or reform the language, including spelling reform, have usually met with failure.
The only significant exceptions were the reforms of Noah Webster which resulted in many of the differences between British and American spelling, such as center/centre, and dialog/dialogue. (Other differences, such as -ize/-ise in realize/realise etc, came about separately.)
Besides the quirks the English spelling system has inherited from its past, there are other idiosyncrasies in spelling that make it tricky to learn. English contains 24 separate consonant phonemes and, depending on dialect, anywhere from fourteen to twenty vowels. However, there are only 26 letters in the modern English alphabet, so there cannot be a one-to-one correspondence between letters and sounds. Many sounds are spelled using different letters or multiple letters, and for those words whose pronunciation is predictable from the spelling, the sounds denoted by the letters depend on the surrounding letters. For example, the digraph "th" represents two different sounds (the voiced interdental fricative and the voiceless interdental fricative) (see Pronunciation of English th), and the voiceless alveolar fricative can be represented by the letters "s" and "c".
There was also a period when the spellings of words were altered in what is now regarded as a misguided attempt to make them conform to what were perceived to be the etymological origins of the words. For example, the letter "b" was added to "debt" in an attempt to link it to the Latin debitum, and the letter "s" in "island" is a misplaced attempt to link it to Latin insula instead of the Norse word igland, which is the true origin of the English word. The letter "p" in "ptarmigan" has no etymological justification whatsoever.
Furthermore, in most recent loanwords, English makes no attempt to Anglicize the spellings of these words, and preserves the foreign spellings, even when they employ exotic conventions, like the Polish "cz" in "Czech" or the Old Norse "fj" in "fjord" (Although New Zealand English exclusively spells it "fiord"). In fact, instead of loans being respelled to conform to English spelling standards, sometimes the pronunciation changes as a result of pressure from the spelling. One example of this is the word "ski", which was adopted from Norwegian in the mid-18th century, although it didn't become common until 1900. It used to be pronounced "shee", which is similar to the Norwegian pronunciation, but the increasing popularity of the sport after the middle of the 20th century helped the "sk" pronunciation replace it.
The spelling of English continues to evolve. Many loanwords come from languages where the pronunciation of vowels corresponds to the way they were pronounced in Old English, which is similar to the Italian or Spanish pronunciation of the vowels, and is the value the vowel symbols [a], [e], [i], [o], and [u] have in the International Phonetic Alphabet. As a result, there is a somewhat regular system of pronouncing "foreign" words in English, and some borrowed words have had their spelling changed to conform to this system. For example, Hindu used to be spelled "Hindoo", and the name "Maria" used to be pronounced like the name "Mariah", but was changed to conform to this system. It has been argued that this influence probably started with the introduction of many Italian words into English during the Renaissance, in fields like music, from which come the words "andante", "viola", "forte", etc.
As examples of the idiosyncratic nature of English spelling, the combination "ou" can be pronounced in at least eleven different ways: "famous", "journey", "cough", "dough", "bought", "loud", "tough", "should", "you", "flour", "tour"; and the vowel sound in "me" can be spelt in at least eleven different ways: "paediatric", "me", "seat", "seem", "ceiling", "people", "chimney", "machine", "siege", "phoenix", "lazy". (These examples assume a more-or-less standard non-regional British English accent. Other accents will vary.)
The state of English spelling
It has been shown that regular alphabetic spelling systems make languages easier to learn. Indeed, the concept of learning "spelling" seems very strange to literate speakers of languages such as Finnish or Spanish, as those languages' spelling systems are extremely regular. This is also the case with several abugida alphabets, such as Devanagari, used to write many languages of India. Vietnamese used to be written exclusively using Chinese characters, so that becoming literate in Vietnamese required years of study, and as a result, very few people were literate. However, after the introduction of a modified form of the Latin alphabet for writing Vietnamese, the writing system could be mastered by a native speaker with only a few hours or days of study, and literacy in Vietnamese is much more widespread now. English, it seems, is somewhere in between: its spelling system is highly irregular, but it is regular to some degree and mastery only requires knowledge of the 26 letters of the alphabet, whereas mastering written Chinese or Japanese is much more difficult, requiring the memorization of thousands of different characters. Studies have shown that dyslexia occurs more often (or at least is more noticeable) among speakers of languages such as English whose orthography differs heavily from the phonology than speakers of languages where the letter-sound correspondence is more regular.