Radak: A man's way is not clear via learning alone, without deed. Just like one must train a youth to learn, so one must train him in deeds ¹ from when he is small, to guard what he learns. If so, he will depart from childish ways, and all the more so when he is mature; his path will always be clear.

Malbim: Verse 1 said that the pure path is only via Torah. Here it adds that even if he can make his Derech clear, he cannot do so for Archo. Derech is the big fixed path. Orach is what branches off the big road and goes to the side. The big path is the way of mercy, generosity, humility and similar [Midos] that are planted in all souls. A person can find it with his intellect, but it is Tamim only via Torah. The small paths, one cannot know them at all from his thinking, e.g. to whom to be generous, and how much; how to conduct with this at all times, in every place and matter. Especially a youth can know this only via Torah!

Radak: One must guard like he learns. If he does not, his path is not clear, rather, ugly.