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I have not seen な used with a noun in this manner before (I know it can be used with な "nouns" but I do not believe that 日曜日 is one of these) and was wondering what is the exact effect of な in this passage and if there are any particular grammar rules around its usage here.
今日は日曜日なので銀行は休みです: Banks are closed because today is Sunday.
Any help would be much appreciated.
Much as 古手梨花 described, ので is used to express a reason for something, similar to "because" in English. Grammatically, ので is the conjunctive conjugation of のだ (plain form) and のです (polite form), which can be parsed as possessive or genitive の + the copula ("to be") verb だ or です.
の attaches directly to nouns or noun phrases, but because のだ describes reasons, it attaches to statements. If you're making a statement about a noun, you finish with the copula だ or です. And, grammatically, those statements that のだ attaches to must be in the 連体形【れんたいけい】 or "attributive" form, and the attributive form of だ or です is な.
Examples:
今日は日曜日なので銀行は休みです。
Basically, this is a longer sentence with a shorter sentence embedded. Let's break this down like the simpler example further above.
「ので」 is used when we express resons (理由・原因). Connection (接続) of 「ので」 is 連体形 + の.
［接助］《準体助詞「の」＋格助詞「で」から》
「ので」 comes from 準体助詞「の」 plus 格助詞「で」, which means 「の」 could be seen as a noun here.
You might think 「日曜日のので」 is the right answer, but it's just too difficult to say in daily converstion (because of the double の, it sounds weird), so we use 「日曜日なので」 instead.
