I kind of talked about this in "#9. Creating New Words," but creation and change, though connected, are different.
The types of changes languages undergo are:
1) Lexical changes (word creation and extinction)
2) Phonetic and phonological changes (changes in pronunciation)
3) Spelling changes
4) Semantic changes (changes in meaning)
5) Syntactic change (changes in sentence structure)
As you can see, there's more to language change than just new words. So I decided on this topic for this post. :)
Alexandra left me a question on my post #9: Do you have any ideas as to why some languages (like English or Korean) might change so rapidly, while others stay relatively static?
She asked this because she had read an article about Icelandic speakers not having any trouble reading 14th-century texts because their language hasn't changed much. The National Science Foundation also says that Japanese has changed relatively little over 1,000 years, while English has evolved rapidly in just a few centuries.
So I'm curious about her questionm, too. I certainly don't have the correct answer to her question, but what I have researched might partly answer her question.
The consensus seems to be that languages change. That much is obvious to most people. Then, the question most people are probably curious about: WHY do languages change? Professor Chris Pontain at the University of London says this is "one of the big questions in modern linguistics which has still only received a partial and tentative answer."
In the 18th century, the theory was that language "decays" because people are lazy. The theory of the late 19th century was that language naturally changes little by little, and it isn't in our (humans') control. The recent theory advocated by American linguist William Labov is that because people are social animals, we influence each other, and language changes as a result. For example, if I start calling a pen "frindle" and my friends start calling it that too and it eventually spreads throughout the whole country. There are problems with all three of these theories, but the last one is the most believable one to me.
These are the general theories, and there are a lot of causes behind language change, too, such as colonization, new technologies, etc. I talked about some of this in my other post.
To try to answer Alexandra's question:
As for Icelandic, there is probably a correlation between Iceland's policy of linguistic purism. From early 19th century, Iceland has replaced "loanwords" with new words from Old Icelandic and Old Norse roots so that these foreign words don't infiltrate their language. North Korea has a similar policy, and thus, the Korean there is a lot different from the Korean we speak in South Korea. Korean was also influenced by the Japanese Occupation during World War II.
So, I think government policy and economic advancement have a lot to do with language change. However, further than that, I don't have enough information to determine why Icelandic or Japanese hasn't changed much while others have. But another interesting thing is that although English has changed a lot, apparently, our spelling of words hasn't changed much because it's usually maintained the same even if the pronunciation changes. This makes it easier for us to read literature from “the old days”, but because of other language changes, we still have problems understanding the literature.
So, I come again to the inevitable conclusion about language: we really can't know for sure. But it is still interesting to think about and develop our own theories.