のなら: emphasis or hearsay?


What is the additional nuance in meaning when の is inserted before なら? For
example:

> トムが行く(の)なら、私も行きます。

In the Tobira Gateway to Advanced Japanese (p. 69) it says

> の occurs before なら when the supposition is based on what the speaker has
> heard from someone or learned from the situation.

However, other websites such as <https://maggiesensei.com/2015/11/07/how-to-
use-%E3%81%AA%E3%82%89-nara/> and <https://jlptsensei.com/learn-japanese-
grammar/%E3%81%AA%E3%82%89-nara-meaning/> explain that の adds emphasis.




This の is an [explanatory-の](https://japanese.stackexchange.com/q/5398/5010).
Roughly speaking, it adds the nuance of "(if) ... is the case" or "(if) it's
that ...". This の is usually optional, but it indicates the conditional is
something that does matter now.

You can use の to seek clarification in an interrogative sentence (see
[this](https://www.wasabi-jpn.com/japanese-grammar/question-markers/)). This
connotation is preserved when it's used with なら. If you understand the
difference between "行く？" and "行くの？", you know the difference between 行くなら and
行くのなら, too. This type of の is often used with external information, but の is
not a hearsay marker _per se_. It's not a plain emphasis marker that turns "if
Tom comes" to "if Tom does come", either.

With that said, it's okay to add の unconditionally most of the time in
conversations. The difference is usually very small. の tends not to be used
with "generic fact" or "purely hypothetical" conditionals that have little to
do with the current situation at hand (e.g., コインを投げて表が出るなら, 明日地球がなくなるなら,
サンタクロースが実在するなら, ...).




This の is an [explanatory-の](https://japanese.stackexchange.com/q/5398/5010).
Roughly speaking, it adds the nuance of "(if) ... is the case" or "(if) it's
that ...". This の is usually optional, but it indicates the conditional is
something that does matter now.

You can use の to seek clarification in an interrogative sentence (see
[this](https://www.wasabi-jpn.com/japanese-grammar/question-markers/)). This
connotation is preserved when it's used with なら. If you understand the
difference between "行く？" and "行くの？", you know the difference between 行くなら and
行くのなら, too. This type of の is often used with external information, but の is
not a hearsay marker _per se_. It's not a plain emphasis marker that turns "if
Tom comes" to "if Tom does come", either.

With that said, it's okay to add の unconditionally most of the time in
conversations. The difference is usually very small. の tends not to be used
with "generic fact" or "purely hypothetical" conditionals that have little to
do with the current situation at hand (e.g., コインを投げて表が出るなら, 明日地球がなくなるなら,
サンタクロースが実在するなら, ...).

