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Most of the time (?) a following dummy noun is required when the speaker wants
to put a 用言 where a 体言 is expected, as shown by the sentence below:

> できない **の** をできないというのに、不思議があるもんか。

However, it seems that sometimes a 用言 on its own can function as a 体言, namely,
with no dummy noun added, such as:

> わかるまで **待ってる** がいい。

> できそうもない幾何の問題を持って **迫った** には冷や汗を流した。

So the question boils down to:

When a 用言 is used as a 体言, in what case is the addition of a dummy noun 1)
required, 2) optional, 3) plainly wrong?

All three instances are from the novel
[Botchan](https://www.aozora.gr.jp/cards/000148/files/752_14964.html).
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particle-の particle-こと
Dummy noun, to add or not to add
The attributive form of a verb worked as a nominalized verb in archaic
Japanese.

  * [How can （の）目指すは be grammatical?](https://japanese.stackexchange.com/q/12538/5010)
  * [に越したことはない Wondering about the に](https://japanese.stackexchange.com/q/36095/5010)

In particular, `Verb + が + いい/よい` is a highly old-fashioned and pompous way of
making a request/command. It's mainly used as role language of pompous nobles
today.

  * [<動詞の辞書形> + がよい ― How is this allowed?](https://japanese.stackexchange.com/q/6714/5010)
  * [Plain Verb followed by がよい](https://japanese.stackexchange.com/q/64661/5010)

(待ってるがいい sounds a little unnatural today, though. 待ってる is a fairly colloquial
contraction of 待っている, and がいい doesn't go well with such a colloquial
expression. 待つがいい, 待っているがいい or 待っておるがよい would be more natural.)

However, this type of nominalization works only in fixed idioms, proverbs and
set phrases in modern Japanese. This sentence:

> できそうもない幾何の問題を持って迫ったには冷や汗を流した

...is regarded simply as grammatically incorrect by today's standard. You
simply need の after 迫った. Since this is from an old and famous novel, I imagine
something like this may have been more tolerated in those days, but you should
not do this in modern standard Japanese.
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answer