grammar - Are ここ, そこ and あそこ prepositions?

これ, それ and あれ are demonstrative pronouns. この, その and あの are demonstrative determiners. What could you call ここ, そこ and あそこ? Are they prepositions?

Answer

I'm afraid that English and Japanese lexical categories don't match up quite that well.

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   これ, それ and あれ are demonstrative pronouns.

   
   

   
   

   That works. In Japanese, these are called 指示代名詞【しじだいめいし】 'demonstrative pronouns'.

   
   

   Keep in mind, though, that grammatically they're really more like English nouns―they permit attributive modification, which English pronouns do not. In Japanese grammar, this kind of modification is called 連体（れんたい） "attributive" 修飾（しゅうしょく） "modification".

   
   


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   この, その and あの are demonstrative determiners.

   
   

   
   
   

   Not really. Japanese has nothing like English determiners. These are 連体詞【れんたいし】 'attributive words', a category English does not have. They're a class of non-inflecting function words whose sole function is attributive modification (連体修飾). Functionally, the closest thing English has to この would be attributive-only adjectives like mere, but that's not really the same thing either.

   
   

   In English, nouns permit two very different kinds of premodification. First, they can be determined:

   
   

   
   

   this hat　　この帽子【ぼうし】

   
   

   
   

   And second, they can be modified attributively:

   
   

   
   

   red hat　　赤い【あかい】帽子

   
   

   
   

   But in Japanese, both この and 赤い are the same kind of premodifier, called 連体修飾[語]{ご}. Because this is true, in Japanese, you can say both of the following:

   
   
   

   
   

   赤いこの帽子　　*red this hat
   この赤い帽子　　this red hat

   
   

   
   

   So it doesn't make sense to say that この is a determiner in Japanese, as Japanese doesn't have that kind of premodification. Although it corresponds to an English demonstrative determiner, it is grammatically quite different.

   
   


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   What could you call ここ, そこ and あそこ? Are they prepositions?

   
   

   
   
   

   These are 指示代名詞【しじだいめいし】 (demonstrative pronouns) again.

   
   

   English prepositions are a category of mainly directional words that are (in traditional grammar) transitive, taking noun phrases as objects. The category in Japanese corresponding to English prepositions is a subset of the Japanese particles, which some linguists call postpositions:

   
   

   
   

   東京【とうきょう】からニューヨークへ　　　　from Tokyo to New York

   
   

   
   

   Japanese words like ここ are nothing like this:

   
   

   
   

   ＊東京ここニューヨークそこ　　　*Here Tokyo there New York

   
   

   
   

   This is super ungrammatical―these words can't be used like directional postpositions. Instead, they are functionally more like nouns:

   
   
   

   
   

   ここは、どこですか？　　　　　　Where am I? (lit. "Where is here?")

   
   

   
   

In terms of etymology, you can consider これ and こ as historically being the long and short forms of the same word. The short form こ had relatively restricted distribution, appearing mainly in compounds and before the genitive particle (almost always の). This was true of the other demonstratives as well, and of the old personal pronouns as well (e.g. long form われ and short form わ), although in the case of personal pronouns the other genitive, が, was generally used instead of の (giving わが, not わの).

In modern Japanese, この and わが are now considered single attributive words (連体詞【れんたいし】), but you can still understand them as the combination of a noun plus a genitive particle if it helps you understand how they work grammatically.

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Notes:

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   For more details about the etymology, see Frellesvig's A History of the Japanese Language (2010) starting on page 136.

   
   


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   Another common (and more literal) translation of 連体【れんたい】 is "adnominal". In Japanese, the term comes from 体言【たいげん】に連なる【つらなる】, and it literally expresses a relationship with a following nominal (independent non-inflecting word―basically, nouns). Compare 連用【れんよう】 and 用言【ようげん】, and the inflectional forms 連用[形]{けい} and 連体[形]{けい}.