Rashi on Deuteronomy 12:28/רש"י על דברים י״ב:כ״ח

1שמר KEEP [AND HEAR (understand) ALL THESE WORDS WHICH I COMMAND THEE] — This (the word שמר) implies the studying of the oral law — that you must keep it within you that it should not be forgotten, just as it is said, (Proverbs 22:18) “[And apply thy heart unto my knowledge] for it is a pleasant thing if thou keep (תשמרם) them within thee”, and only if thou learnest is it possible that thou wilt understand and fulfil the commands, but one who is not amongst those who study (lit., who is not included in learning) cannot be amongst those who act correctly (cf. Sifrei Devarim 79:3; see also Rashi on Deuteronomy 4:6). (The translation is: keep, i.e. study and retain your learning in mind: ושמעת, and then thou wilt understand how to observe the commands).

2את כל הדברים [KEEP …] ALL WORDS [WHICH I COMMAND THEE] — This implies that a light precept should be as dear to you as a grave precept (Sifrei Devarim 79:4).

3הטוב [THAT IT MAY GO WELL WITH THEE … WHEN THOU DOEST] WHAT IS GOOD — This refers to an action that is proper in the eyes of the Heavenly Father.

4והישר AND WHAT IS RIGHT — this refers to an action that appears to be proper in the eyes of men (Sifrei Devarim 79:5).