What causes loan words to displace native words for existing concepts?


I've been told several times that things like _class_ and _classmate_ don't
have non-loan words (I don't know what those are called) associated with them
in Japanese. This seems ludicrous to me, as the ideas _must_ have existed
since well before the loan words were ever heard. So what causes loan words
such as クラス to become so popular?




Actually there are several English-origin words that effectively took the
place of traditional Japanese words in the last 100 years or so. (Hereafter,
_traditional_ words mean both 漢語 and 和語—whatever that can safely be written in
hiragana and/or kanji)

For example, traditional Japanese words such as 暦【こよみ】, 匙【さじ】 and 厠【かわや】 are
very rarely used in daily conversations today. Instead, we use カレンダー, スプーン,
and トイレ, respectively, some of which are even longer and more difficult to
pronounce than the original words. Of course the concepts of those words are
universal.

One cause for this is probably related to the Japanese education system. Those
kanjis are a bit complicated and not listed in
[教育漢字](http://ja.wikipedia.org/wiki/%E5%AD%A6%E5%B9%B4%E5%88%A5%E6%BC%A2%E5%AD%97%E9%85%8D%E5%BD%93%E8%A1%A8),
and not taught in elementary schools. Two of them are even not in 常用漢字表, so
many Japanese grow up without learning how to write them. Gradually over time,
such situation can make people hesitant to use traditional words, and use
English-origin alternatives.

That said, I doubt クラス replaced any Japanese traditional words. The word クラス
has several meanings, but generally it can safely be translated into
traditional Japanese words which are still in use today , such as 学級【がっきゅう】,
組【くみ】, 階級【かいきゅう】, or 授業【じゅぎょう】.

