Page 682 - Makino,Tsutusi.DictionaryOfIntermediateJGrammar

608 zaru o enai

When Vneg nakereba naranai is used in a context where one has to do
s.t. because she has an obligation to do it, it cannot be replaced by
Vneg zaru o enai, as shown in [2]. In [2] the writer had made a promise
with S.O. to go see a movie and as a result of that had to leave home
right away. In such a case Vneg zaru o enai cannot replace Vneg
nakereba naranai.

But if the same predicate is used in a situation where there is no choice
but to leave a place right away, Vneg zaru o enai can replace Vneg
nakereba naranai, as shown in [3].
L+ Zri
[3] Tt~*bD%%i?71:0~, tCttl$lf {&lfhldl&%&fi>~~
f< / g6%fs&fi'7f:l0
(Because it was the president's order, I had to leave right away.)

The crucial difference between the two structures is: Vneg zaru o enai is
used to express only a no-choice situation but Vneg nakereba naranai is
used to express either a no-choice situation or an obligation situation.
(+ - nakereba naranai (DBJG: 274-76))

11. Vneg zaru o enai can be also replaced by Vneg wake ni (wa) ikanai and
V shika nai as shown in [4].

Vneg wake ni (wa) ikanai can be used in both no-choice and obligation
situations, and implies the speaker's unwillingness to do s.t. indicated