Currently I am studying the ~ので,。。 grammar and, according to the book, I should use the short form before it, such as in もっと勉強したので、頭が痛いです。
Then, when I have a な-adj or noun, can I write it as follows for the presente tense ??:
元気だので、外に行きます。instead of 元気なので、外に行きます。which is the correct form according to the text book? or is it wrong?
Not sure what you mean by the 'short form'. I'm guessing you mean the dictionary form rather than masu form. So, as you say, 勉強したので is correct.
元気だので is wrong. In the non-past tense nouns and na-adjectives both take な; never だ. So you would have 元気なので, 先生なので etc.
However, in the past tense you would still use だった; there is no replacement with な in this case. So 元気だったので, 先生だったので etc.
