Why is の needed in 行くのでしょうか but it is not needed in 行きますでしょうか?


> １．行く **の** でしょうか  
>  ２．行きますでしょうか

Why is の needed between 行く and でしょうか in the first question, but there is no
particle between 行きます and でしょうか in the second question?

Context: both questions come from the [this
video](https://youtu.be/_oMtifQpT4Q?list=PLOcym2c7xnBwU12Flkm5RcLIEhvURQ8TB&t=780)
aimed at Japanese learners (see minute 13:00).




First

  * 行くのでしょうか
  * 行くでしょうか

are both possible but 行く **ん** でしょうか would be more common in most contexts.

Second,

  * 行きますでしょうか

is acceptable in speech but technically wrong as pointed out by @Nameless,
which can be felt more strongly in the plain form

  * 行きますです

Third,

  * 行きます **の** でしょうか

is acceptable to me, but slightly odd. Since 行きますでしょうか is already irregular,
there may be not much logic for this, but one thing could be that this の is a
nominalizer, which is not usually used after ます.

* * *

Grammar related to this の.

  * ん in んです is [格助詞「の」の音変化](https://dictionary.goo.ne.jp/word/%E3%82%93/#jn-239154)
  * The only [格助詞 definition of の](https://dictionary.goo.ne.jp/word/%E3%81%AE/#jn-171157) which can follow conjugatable words is 準体助詞 (5-2 in the link).

Another thing is

  * です follows directly a conjugatable word only in 未然形 (so 行くです is wrong, 行くでしょうか is fine). ([補説 here](https://dictionary.goo.ne.jp/word/%E3%81%A7%E3%81%99/#jn-151501))

