Why is there a 'な' before ’ので’ in this sentence?


このバックパックはよさそう **な** ので、買うつもりです。

'Because this backpack looks good, I intend to buy it.'

Following ~sou there is a 'na' placed before the 'node'. Why is this the case?
Is it because of the ~sou implemented in the sentence? Furthermore, would the
sentence still be grammatically correct even if the 'na' was not there?




> 「このバックパックはよさそう **な** ので、買{か}うつもりです。」

「ので」 can only attach to the 連体形{れんたいけい} ("attributive form") of a conjugated
word. It simply cannot attach to any other conjugated form by "rules".

「そう **な** 」 is the 連体形 of the auxiliary verb 「そう **だ** 」; therefore, that is
the **_only_** correct form that can directly precede 「ので」.

Thus, you **_cannot_** ever say 「そう **だ** ので」、「そう **だろ** ので」、「そう **で** ので」、「そう
**に** ので」 or 「そう **なら** ので」.

> Furthermore, would the sentence still be grammatically correct even if the
> 'na' was not there?

No, it never would. 「そうので」 is simply ungrammatical because, as I stated above,
「そう」 is not the 連体形 of a word.

