(A) は (B) となっている is a common expression used to show that ‘(A) is becoming (B)’, or (more commonly) ‘(A) has become (B)’. This expression will only appear with nouns, or
な-Adjectives in their 語幹(ごかん) (stem form). (A) は (B)
となっている is often translated as ‘(A) happens to be (B)’, ‘(A) has been established as (B)’, or ‘it has been decided that (A) is (B)’.
Let’s have a look at some examples, and then how this structure is formed.
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鬼滅(きめつ)の刃(やいば)は非常(ひじょう)に人気(にんき)となっている。
It happens to be that Demon Slayer is very popular.
日本(にほん)ではドラッグは法律上(ほうりつじょう)、違法(いほう)となっている。
In Japan, it has been established that drugs are illegal, by law.
このプールでは、飛(と)び込(こ)みが禁止(きんし)となっている。
At this pool, it happens to be that jumping in is not allowed.
Despite the different translations, (A) は (B)
となっている always has the same meaning, which can be understood as follows:
(A) は - (A) is a constant unchanging thing.
(B) と - (B) is a result. (coming form the 格助詞(かくじょし) (case marking particle) と’s role in highlighting results.
なっている - (A) is existing in the state ‘having arrived at the result of (B)’.
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今(いま)の時代(じだい)にはインターネットは必要(ひつよう)となっている。
It is now that the internet is necessary in the present age.
この高校(こうこう)ではアルバイトが禁止(きんし)となっている。
It is now that working part-time at this school is prohibited.
Due to this, the literal translation may be thought of as ‘it is now that (A) is (B)’ in almost all situations.
(A) は (B)
となっている may be rephrased to (A) は (B) になっている. However,
に will remove much of the feeling of a ‘result’, or ‘finiality’. Due to this, it will just sound like (A) is becoming (B), but is not necessarily that way yet.
In writing, おる is not always considered a ‘formal’ equivalent of いる, especially in set phrases.
となっている is often seen as となっており, instead of となっていて. This is particularly common in news articles.