Based on what the other person has said, the speaker makes a proposal or gives his/her opinion or suggestion about what to do under those circumstances. Such patterns as ～ましょう(Let's ～), ～てください(Please ～), and ～がいいですよ(～would be good) often come at the end of the sentence. Vなら can be replaced by それなら.
V is either in the dictionary form or in the Vない form.
A: 北海道へ行きたいとおもっています。
I would like to go to Hokkaido.
B: →北海道へ行くなら、ひこうきが便利ですよ。
If you are going to Hokkaido, it is convenient to go by plane.
A: 夏休みには国へ帰らないつもりです。
I don't plan on going back to my country during summer vacation.
B: →国へ帰らないなら、わたしの家へあそびに来てください。
If you are not going home, please come over to my house.
B: →それなら、わたしの家へあそびに来てください。
If that's the case, please come over to my house.
