A modern perspective
〜ければ is the conditional ending for adjectives. Since the
〜ない form of verbs is shaped like an adjective, it uses adjective endings like
〜ければ. That's it! From a modern perspective, there are no steps in-between.
However, some grammars do take it one step further: they divide
〜けれ (the hypothetical form of an adjective), plus
ば (a conditional particle). This explanation is attractive because
ば appears elsewhere with conditional meaning, as in
But either way, it's pretty simple.
〜なければ is the negative conditional form.
A historical perspective
〜ければ was presumably formed by a contraction of some sort. But what sort? The explanation @istrasci gives is a common one and can be found elsewhere on this site, that
〜ければ was a contraction of
〜く+あれば. In turn, we can say
あれば was a form of
ある with the conditional
However, this leaves us wondering why the first vowel is /e/ rather than /a/. Sansom, on page 205 of his 1928 Historical Grammar of Japanese, gives an explanation:
This change is easy to understand, for the final e of kare influences the preceding vowel a, by a tendency which is common in Japanese.
In contrast, Frellesvig, in his 2010 History of the Japanese Language, suggests that the
〜けれ stem comes from a contraction of *ki-are into kyere, giving way to modern kere. Either way, it's a contraction of some sort of adjectival form plus
What is ある doing there?
The presence of
ある is most likely purely grammatical, inserted so that endings like
〜ば can attach to adjectives. We shouldn't really translate it as "exists" in English. Instead, we should describe
〜なければ as the conditional form of the negative
This sort of grammatical use of
ある is common and can be seen in all sorts of places in Japanese, including in other adjectival inflections:
〜なく＋あった ＞ 〜なかった
I trust you understand that
〜なかった is simply a negative past form, and that you don't attempt to translate it as though it has "exists" in it. You can apply the same sort of logic here.
Historical versus modern
Why look at it from a modern perspective? Well, let me ask you this: when you say goodbye in English, do you always think God be with ye? Of course not! Historically speaking, it's a contraction of that phrase, but today speakers use goodbye as a pre-formed unit. It's simply part of their vocabulary. In that sense, the word is no longer a contraction.
Likewise, when you say
〜なければ, please don't think something like "if it's such that you don't", as @istrasci wrote. It's simply a negative conditional form.
†For simplicity's sake, I'm referring to this verb as ある, which is its modern citation form. Its historical citation form was あり.