未然形 of である and 連用形 of だ


When I checked the entry of 助動詞 「だ」 in the
[dictionary](https://dictionary.goo.ne.jp/jn/132198/meaning/m0u/), I found the
following example sentence:

> 「学生は怠けるべき **で** はない」

Does the fact that で is in bold mean that で in ではない is actually the 連用形 of
「だ」?

But how could this be possible considering the fact that ではない is, in itself,
the negative form of である ? Which is what 「だ」 actually comes from ?

Are there 2 ではない ?




だ and である are really two forms of the same verb, which is a very odd and
irregular verb. There's not really a benefit to thinking of them as separate,
as every other form besides です～であります is shared between the two. Having two
options for the shuushikei is only one way this verb is unusual.

In terms of the mizenkei, I'm not sure it even has one. で is the ren'youkei,
but ではない as a unit (sometimes without the は as でない) is the negative form,
which isn't formed off of any mizenkei, it simply is on its own the negative
form. Etymologically, it is from the ren'youkei + は + ない, much the same way as
adjectives are topicalised and negated - c.f. 早くはない. (でない, without the topic
marker, is formed in exactly the same way as adjective negatives are.) At this
point, though, it's basically a unit, and there's no sense in arguing that
it's still used as a multi-part construction.

To answer your questions directly:

  * Yes, technically, putting で in bold there is demonstrating its use as a ren'youkei, but that's not really a helpful way of thinking about ではない anymore. It probably would have been better to put the whole word ではない in bold.
  * There are not two ではないs, because だ and である are the same verb. ではない is an irregular negative form that's built off a ren'youkei + topicaliser combination, rather than a mizenkei (or an adjective-like plain ren'youkei).

