"今日は先生が休みなので、授業がなくなった," the phrase "なので" is used to connect the cause (the teacher is on break) with the effect (the class was canceled). Let's break down why "なので" is necessary here:
Explanation:
"ので" by itself:
"ので" is a conjunction that means "because" or "since," and it’s used to indicate a reason or cause. It’s typically used to connect two clauses where the first clause provides the reason for the second.
However, "ので" is usually preceded by a plain form of a verb, adjective, or noun.
Why "な" is needed before "ので":
When you want to use "ので" after a noun or a na-adjective (形容動詞, keiyō dōshi), you need to add "な" before "ので" to make the connection grammatically correct.
In your sentence, "休み" (yasumi) is functioning as a noun, and "先生が休み" describes the current situation. Since you’re connecting this noun phrase to "ので," you must insert "な" to form "なので."
Example Breakdown:
先生が休み (Sensei ga yasumi) — The teacher is on break.
なので (nanode) — Because of that, or since that’s the case.
So, "今日は先生が休みなので、授業がなくなった。" means "Since the teacher is on break today, the class was canceled."
What if you use just "ので"?
If you used only "ので" without "な," the sentence would be grammatically incorrect when connecting to a noun or a na-adjective, as the correct connector "な" is required.
Summary:
Use "なので" when connecting a noun or na-adjective to "ので" to indicate reason or cause.
Use "ので" directly after verbs and i-adjectives in their plain form.