What is the difference between the negative forms -ず and -ぬ?


-ず and -ぬ are two alternatives to the negative form -ない / -ません. But I noticed that depending on the word, it's either -ず or -ぬ, although it seems like some words can take both suffixes. Some examples I have encountered:

# -ず

知らず (lyric in song "タッチ" by Younha)  
わからず (lyric in song "愛するもの" by Angela Aki/アンジェラ・アキ)  
せず (song title: "油断せずに行こう")  
なれず (lyric in song "未来へ" by Kiroro/キロロ)

# -ぬ

気づかぬ (lyric in song "ありがとう..." by KOKIA)  
届かぬ (song title: "届かぬ想い")  
ならぬ (lyric in song "春雷" by YeLLOW Generation)

# both -ず and -ぬ

とらず / とらぬ  
できず / できぬ

So are there rules for choosing between -ず and -ぬ?




In the modern form, ず is only used as an adverbial (食べずに出る leave without
eating). ぬ can replace ない.

In 文語, the grammar used in writing until the reformations after WWII and still
at least partially in many forms of poetry, songs, and very formal documents,
the use of ず and ぬ was/is grammatically constrained in a manner no longer
present in modern Japanese. ぬ was used with the 連体形 form of a word, meaning
that it connected with a following noun (and it still does that).

ず, on the other hand, connected with the 未然形 and was used for negative
assertions. It had conjugations, too, but those are really really rare now
(ざら、ざり、ざる、ざれ、ざれ).

Just remember this: ず can end a sentence, but ぬ can't; ぬ modifies nouns, but ず
doesn't: 知らぬ人、人知らず.

Of course they've also snuck into idioms and 慣用語: 我知らず (despite
oneself)、暑さ知らずの所 (a place that knows no heat), 知らぬ顔 (pretend
ignorance)、知らぬ存ぜぬで押し通す (persist in asserting one's innocence). (新和英大辞典)




In the modern form, ず is only used as an adverbial (食べずに出る leave without
eating). ぬ can replace ない.

In 文語, the grammar used in writing until the reformations after WWII and still
at least partially in many forms of poetry, songs, and very formal documents,
the use of ず and ぬ was/is grammatically constrained in a manner no longer
present in modern Japanese. ぬ was used with the 連体形 form of a word, meaning
that it connected with a following noun (and it still does that).

ず, on the other hand, connected with the 未然形 and was used for negative
assertions. It had conjugations, too, but those are really really rare now
(ざら、ざり、ざる、ざれ、ざれ).

Just remember this: ず can end a sentence, but ぬ can't; ぬ modifies nouns, but ず
doesn't: 知らぬ人、人知らず.

Of course they've also snuck into idioms and 慣用語: 我知らず (despite
oneself)、暑さ知らずの所 (a place that knows no heat), 知らぬ顔 (pretend
ignorance)、知らぬ存ぜぬで押し通す (persist in asserting one's innocence). (新和英大辞典)

