Kommentar zu Dewarim 4:1

ועתה ישראל, after you have seen the decree of G’d to exile you if you sin, be on guard to meticulously observe His commandments without adding to them or detracting from them. An addition or a detraction will both have destructive consequences for you.

ועתה ישראל שמע אל החוקים, "And now, O Israel, hearken to the statutes, etc." Perhaps Moses referred to 2 separate incidents which had happened to him personally (when he had been remiss.) The first instance was the matter of speaking to the rock when Moses had struck the rock instead in his eagerness to fulfil G'd's instructions. The second instance was the matter of Zimri publicly cohabiting with the Midianite princess Cosbi when Moses had forgotten the ruling that anyone sincerely jealous of such a desecration of the name of the Lord is to execute the sinner forthwith without trial. On that occasion too, Moses had missed an opportunity to sanctify the name of the Lord as we have explained in that context. Even though Pinchas had articulated this halachah in front of Moses at the time (compare Sanhedrin 82) Moses had answered him: "he who reads the letter must execute what is written therein." As a result, Pinchas acquired the merit of the covenant of peace which in truth had been reserved for Moses himself.

ועתה שמע ישראל, “Now, O Israel, listen!” after Moses had concluded the words of admonition and reminded them of their various past sins, all of which had caused negative fallout for them, so much so that only the second generation had been allowed to enter the Holy Land, he hints that their entry into this land was contingent on their not copying the mistakes of their fathers. Moses especially warns the people not to tamper with G’d’s commandments by either adding or subtracting from them. [A major insult of the Creator, Who does not need us, the creatures, to edit His work. Ed.]