ねば in 食べねば; relation between ねば and なければ


I would like to know a bit more about the "ねば" grammatical construction. Every
now and then, I hear people say sentences like

> 東京に行かねばならぬ
>
> 粘々丼を食べねば倒れる
>
> なんとかせねばいい

It's quite clear that ねば means the same as なければ, but

  * Is it a shorten form of なければ, or a different construction?
  * Is it an old form, or something that has been around for 20 years or so?
  * Is its usage limited to some regions? ages? social classes?
  * How (in)formal is it?
  * Is it like なきゃ and なくちゃ? (as far as usage is concerned, not nuances)




ねば can be divided into two parts:

  * ね, the 已然形 (realis) form of the Classical Japanese auxiliary verb ず
  * ば, basically the same as modern ば

So to answer your questions in detail (great format, by the way!):

  * It is a different construction -- actually なければ is a reinvention of the same concept (已然形 + ば) using modern parts!
  * It is a very old form, here it is in the Manyoshu: "... 君が目見ねば苦しかりけり" ("... Not seeing you (lit. your eyes), I suffer." -- note the use of the 已然形 was a bit different back then. This changed a few hundred years ago (IIRC) to something more like our usage)
  * I don't think its usage is especially limited except insofar as it is a bit of an archaism. (Note that the first two examples include ならぬ and 倒る, also archaisms.) I think most people would recognize it, but few would use it except for intentional effect (intentionally overformal speech as joke, etc.). However, this kind of pseudo-archaic talk is quite popular among certain internet circles, so it might be more common and "normal" there, and might have seeped out into real life from that point. I hope that someone else will be able to provide better info on this point.
  * As above, it is a bit of an archaism and so while not informal as such, it might be considered inappropriate or unusual in a case where standard Japanese was expected. 
  * なきゃ and なくちゃ are normal standard Japanese, just informal (as you know of course), so it is not like them. 




ねば can be divided into two parts:

  * ね, the 已然形 (realis) form of the Classical Japanese auxiliary verb ず
  * ば, basically the same as modern ば

So to answer your questions in detail (great format, by the way!):

  * It is a different construction -- actually なければ is a reinvention of the same concept (已然形 + ば) using modern parts!
  * It is a very old form, here it is in the Manyoshu: "... 君が目見ねば苦しかりけり" ("... Not seeing you (lit. your eyes), I suffer." -- note the use of the 已然形 was a bit different back then. This changed a few hundred years ago (IIRC) to something more like our usage)
  * I don't think its usage is especially limited except insofar as it is a bit of an archaism. (Note that the first two examples include ならぬ and 倒る, also archaisms.) I think most people would recognize it, but few would use it except for intentional effect (intentionally overformal speech as joke, etc.). However, this kind of pseudo-archaic talk is quite popular among certain internet circles, so it might be more common and "normal" there, and might have seeped out into real life from that point. I hope that someone else will be able to provide better info on this point.
  * As above, it is a bit of an archaism and so while not informal as such, it might be considered inappropriate or unusual in a case where standard Japanese was expected. 
  * なきゃ and なくちゃ are normal standard Japanese, just informal (as you know of course), so it is not like them. 

