While teachers can be pretty sure that one and one will always make two, and that the capital of France will probably continue to be Paris, there's far less certainty when it comes to correcting English.
Language is like a stream. It moves on continually and, like a stream, will always take the easiest route. Thirty years ago, I was marked down for grammatical errors that today seem to have been totally excised. For ease of communication we now happily end our sentences with prepositions and the conditional tense has all but disappeared.
In the 1955 song "He's a Tramp" Peggy Lee sang: "But I wish that he were double" whereas today Boyzone can get away with "If only tears were laughterIf only night was day", making it seem pedantic to mark children down for failing to observe that "were" is obligatory in conditional clauses, regardless of whether the subject is singular or plural.
In recent years English has also abandoned the accusative, genitive, dative and ablative forms - most notably by losing "whom" in its various forms (of whom, to whom and by whom). Similarly, the historic tenses of strong verbs are being eroded. No one says "dove" (dived) "snook" (sneaked) or "clomb" (climbed) any more, and "stank" is becoming displaced by "stinked".
Any rule book about grammar is only ever going to be a snapshot of where our language is at the moment. If we look back to the dictionary which Samuel Johnson published in 1755, it will seem strange to realise that words like "wobble" and "budge" were specifically excluded. As editor, Johnson considered both these verbs to be Americanisms which replicated perfectly adequate words already in English.
"Fun" was also disallowed by Johnson for similar reasons. Eventually all three words found their way into English because, as the language moved forward, people discovered a need for them. Verbal communication will always be pragmatic. French purists tried to stop le weekend finding its way into their language but the country needed it. Similarly, at the moment they are lobbying desperately for un pirate informatique rather than un hacker.
In the 1980s, Welsh scholars tried to remove "peach" on the grounds that it was an English word that should never have found its way into the language. As an alternative they came up with the linguistically-pure eirwen gwlanog (woolly plum), but Welsh speakers ignored such a redundant mouthful and took the easier route.
Other words remain but undergo fundamental changes in meaning. "Nice", for instance, started off meaning "foolish" in Chaucer's time (from the Latin nescius, "not knowing") but by Shakespeare's time, it had come to mean "fastidious", having passed through the intermediate meanings "wanton" (14th century), "strange" (15th century), and "shy" (early 16th century). In the 19th century people talked of "nice" distinctions being exacting ones, but by the 20th century a person who was "nice" was neither foolish, fastidious, wanton, strange or shy, but agreeable. These days "nice" is beginning to have pejorative qualities. Girls today talk of "a nice young man, puh-lease". By the end of the 21st century we may find that the word will mean foolish again.
Attempts to simplify English spelling began almost as soon as Johnson standardised it 250 years ago. Noah Webster, in his American Dictionary, opted to drop the silent letter "u" from words such as favour, honour and harbour (but not from court). Webster also revised words taken into American from the French. Thus metre and centre became meter and center, thereby better reflecting how each is pronounced.
Similarly "quay" became "key" (as in the Florida Keys), although this in itself reflects a curious shift in orthography. In medieval English the word was originally "key" or "keye" but the Plantagenet fashion for adopting all things French changed it to "quay" in emulation of quai.
There is a very good argument these days for rationalising English. Our young people struggle with the illogicalities of this language in a way that simply does not occur in Italian, Welsh or German, for instance. The simple rule of pronouncing whatever letters are written in front of you seems to have eluded the English, which is why our children have such difficulty in remembering to write "through" rather than "thru" and "Featherstonehaugh" instead of "Fanshaw".
How much longer will the English stream try to flow uphill? In the 19th century, the Society for Pure English frequently proposed following Webster's lead, and in 1950 George Bernard Shaw left most of his fortune to try to get a phonetic alphabet implemented for English. In 1953 a bill actually came before the House of Commons for simplified spelling but it failed.
What governments can't manage is often left for the people to achieve. That is always an ongoing process with our language but at the moment technology is accelerating the process rapidly.
According to Crispin Thurlow, of the Centre for Language amp; Communication at Cardiff University, contemporary English is being "influenced by the physical constraints of technology and shifting social-economic priorities". Dr Thurlow has identified "netlingo" as one potent new force in language development.
"The emphasis in netlingo is on a mixture of speed, efficiency, informality and creative typology, whereby the traditional rules of grammar and style are subverted. To this end, netlingo relies principally on expression which is QWERTY-driven, (that is, whatever is possible with the computer keyboard) and economic - saving a keystroke wherever possible."
Netlingo's impact can be seen in various changes that are overtaking contemporary English. Dr Thurlow refers to "lexical compounds and blends, for example, weblish, netiquette and e- or cyber- anything", also to abbreviations and acronyms, for example, THX for "thanks" or F2F for "face to face". There is minimal or no capitalisation, and onomatopoeic or stylised spelling, such as "cooool", "hahahaha" or "vewy intewestin".
How can there ever be a textbook for something that is undergoing continual, not to say accelerating, change? We are each of us left to make a judgment. "CU" - the text-messaging form of "see you" - may never be acceptable in an essay but, ultimately, any innovation within English that allows us to communicate unambiguously ought to be tolerated. What else is the purpose of language if it is not for unambiguous communication?