The above is used to bring out the contrast between N1 and N2, which belong to the same category. N2 is focused upon by the use of なら. This is typically used when giving a negative reply to a question and offering an alternative. The order of the two clauses can be reversed. In other words, the clause which contains なら can be placed first, followed by the one with は.
Is Saturday convenient for you?
Saturday is not good for me, but Sunday is fine.
Sunday is OK, but Saturday is not convenient for me.
A: ジュースはありますか。
Do you have juice?
B: ジュ－スはありませんが、水ならあります。
I don't have juice, but if it's water, I have some.
わたしはテニスならやりますが、ほかのスポ－ツはやりません。
I play tennis, but I don't play other sports.
When the phrase Nなら is used with particles such as [place] へ, [place] で, or [person] と, these particles are placed between N and なら.
ほかの所へは行きたくありませんが、京都へなら行きたいです。
I don't want to visit other places, but if it's Kyoto, I want to go.
工場の中では写真をとってはいけませんが、外でならいいです。
You are not allowed to take pictures inside the factory, but if it's outside, it's all right.
Lesson 9-5「わたしはコーヒーは飲みますが、こうちゃは飲みません。」
I drink coffee, but I don't drink black tea.
Lesson 21-3「北海道へ行くなら、ひこうきが便利です。」
If you are going to Hokkaido, it's convenient to go by plane.