In addition to his carefully curated commentaries on the Four Books, we find each passage discussed extensively with his students in the Collected Conversations of Master Zhu (zhuzi yulei 朱子語類). These conversations are invaluable in ascertaining the precise meaning of the commentaries, and reveal other insights.
Kongzi said: Learn and then seasonally practice it—not indeed pleasant? Have friends from far away places come—not indeed happiness? Not known by others, and yet not angered—not indeed a noble man?
子曰：「學而時習之，不亦說乎？有朋自遠方來，不亦樂乎？人不知而不慍，不亦君子乎？」
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Yulei ch. 20, Lunyu 1:1, 1:
When reading the Lunyu 論語 (“Discourses”) one should thoroughly study the first chapter, Xue er 學而 (“Learn, And Then”). If one has an enlightened grasp of this chapter the rest will easily reveal itself.
今讀論語，且熟讀學而一篇，若明得一篇，其餘自然易曉。
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Yulei ch. 20, Lunyu 1:1, 2:
In the case of each verse in the “Learn, And Then” chapter, what comes earlier in the verse speaks to self-cultivation, and what comes later to kin, teachers, and friends.
學而篇皆是先言自修，而後親師友。
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Yulei ch. 20, Lunyu 1:1, 3:
Entering the gateway of the Way involves immersing oneself in its principles and gradually becoming intimate with them, until, over time, they become one with the self. Yet nowadays, people keep the Principles of the Way here and themselves outside, never truly engaging or intersecting with them!
入道之門，是將自家身己入那道理中去，漸漸相親，久之與己為一。而今人道理在這裏，自家身在外面，全不曾相干涉！
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Yulei ch. 20, Lunyu 1:1, 48:
The words of the sages and worthies are plainly stated in the texts. Take for example this saying by Teacher Kong, “to learn, and then frequently practice it.” This directly instructs that whatever one learns, one should regularly apply in practice.
聖賢言語平鋪地說在那裏。如夫子說「學而時習之」，自家是學何事，便須著時習。