The only problem I have with Mr. Slingerland's translation is his translation of 下學而上達 (하학이상달), which he translated as follows: "I study what is below to comprehend what is above." As mentioned in my post HERE, 上達 (상달) means "to report to a superior," not "to comprehend what is above." Therefore, I would like to suggest the following translation:不怨天不尤人. 下學而上達. 知我者其天乎.“I am not bitter toward Heaven, nor do I blame others. I study what is below to comprehend what is above. If there is anyone who could understand me, perhaps it is Heaven?”
“[I] do not (不) resent (怨) Heaven (天) [and] do not (不) blame (尤) others (人). To inferiors (下) [I] teach (學), and (而) to superiors (上) [I] advise (達). [As for] someone who understands me (知我者), perhaps (其) it is Heaven (天乎)?”Notice that I translated 下學而上達 (하학이상달) as follows: "To inferiors (下) [I] teach (學), and (而) to superiors (上) [I] advise (達)."
As I explained in my previous post, when you "report to a superior" (上達- 상달), you convey information or give them advice, both in a respectful manner. The opposite of 上達 is 下達 (하달), which means "to convey information to or to command an inferior," probably without showing much respect. Therefore, both 上達 and 下達 essentially mean "to convey information" or "to teach," but one is done in a polite way and the other is done in a less polite way. The same thing can be said of 下學 (하학) and 上達 (상달).
下學 (하학) means "to teach inferiors," and 上達 (상달) means "to advise or to inform superiors," so both phrases are similar in that they mean "to convey information," but the difference is a matter of etiquette. In other words, you teach inferiors, but you advise superiors. One would not presume "to teach a superior," especially a king, so the more proper expression for teaching a king is "to advise a king." A king has advisers, not teachers. Therefore, 下學而上達 (하학이상달) could be translated more simply as "I teach (下學) and (而) I advise (上達)."
Confucius may have been somewhat frustrated by the fact that people had a hard time understanding his "teachings" and "advice," but he seemed to have been comforted by the thought that, at least, "Heaven" probably understood.