This can be understood in its plain sense. I.e., the word מצוה (lit., commandment) refers to a [whole] category, as if it said, “all the commandments.”

There is also a Midrashic explanation: If you began to fulfill a mitzvah**, complete it, etc.** I.e., the word כל is related to the word, “And Hashem completed (ויכל) on the seventh day, etc. (Bereishis 2:2).” Alternatively, the word כל is related to the word כלה (entire). In other words, you should perform the mitzvah in its entirety — if you begin to fulfill it, complete it. According to this, the word מצוה written in this verse refers to “a single commandment.”

Not to test Him and not to be critical of Him. Rashi is answering the question: Just because Hashem oppresses them will they guard His commandments? [Therefore] Rashi explains the verse as, “Adonoy oppressed them in order to know… will you guard His commandments.” I.e., that you are not to test Him, etc. It is at if the verse says: ...to know if you will be critical of Him after He oppresses you and say, “Why do I need His commandments?”

As they grew, etc. Rashi is answering the question: What is this verse teaching us, that by storing their garments in a chest they did not wear out? Rashi explains: Even though their garments were on them [i.e., they were worn] for forty years, they did not wear out. And the reason for this is because the clouds of Divine Glory, etc. This is the significance of מעליך (on you). I.e., even though they wore the garments.

This usually happens to those who walk barefoot, etc. You might ask: It says in Parshas Ki Savo (below 29:4), “and your shoe did not shred from your foot,” which implies that they wore shoes! The answer is: That verse there refers to the Jews who left Egypt who had shoes — those shoes did not shred. Rashi’s explanation here refers to the Jews who were born in the wilderness who did not have shoes. However Re”m explains that, “This usually happens to those who walk barefoot,” is only describing the type of swelling, but they were not actually barefoot, etc. And according to this explanation, one should not ask: What is the significance of “nor did your foot swell”? Since they did not walk barefoot, is this not obvious? The answer is: Indeed this is so. The verse teaches us that their shoes did not shred from their feet. It should be understood as follows: Your foot did not swell (which usually happens to those who walk barefoot); and this is because you did not have to walk barefoot, since your shoes did not shred.

Olives that produce oil. I.e., there are two species of olives, and one of them does not produce oil. Therefore it needed to write, “oil-olives.” Alternatively, [Rashi’s explanation is] because we say that the Land of Israel is praised for seven items. But if you count, “olive,” and, “oil,” separately, then you will find eight items. Therefore Rashi explains: “Olives that produce oil.” I.e., it is only one item.