The last verse in Parshas Vaeschanan (Devarim 7:11) says:
וְשָֽׁמַרְתָּ֨ אֶת־הַמִּצְוָ֜ה וְאֶת־הַֽחֻקִּ֣ים וְאֶת־הַמִּשְׁפָּטִ֗ים אֲשֶׁ֨ר אָֽנֹכִ֧י מְצַוְּךָ֛ הַיּ֖וֹם לַֽעֲשׂתָֽם:
You shall therefore, observe the commandments, the statutes, and the ordinances, which I command you this day to do.
The Passuk, while exhorting us to follow the Torah, uses 3 different words to refer to various precepts in the Torah: Mitzvah, Chukim, and Mishpatim. The first of that list, Mitzvah, is written in the singular, while the other two are plural. It seemingly makes more sense for it to be plural (since the context of the Passuk seems to be referring to keeping all the Mitzvos), so why does the Torah use the singular version of 'Mitzvah' when the plural 'Mitzvos' would seem to make more sense?
I'm aware that a lot of places (including the quote I have above) still seem to translate 'Mitzvah' in the plural ('commandments'), but that seems to be due to making an inexact translation. The word 'Mitzvah' is clearly singular, so while a contextual, inexact translation could be written as plural, the literal definition is quite plainly singular.
Tanakh: The Holy Scriptures, published by JPS (found on Sefaria) gives a very creative translation of this Passuk which definitely does not fit with the simple definition, but if anyone could find the source for why they translate it in that manner, I guess that could be an answer:
וְשָׁמַרְתָּ֨ אֶת־הַמִּצְוָ֜ה וְאֶת־הַֽחֻקִּ֣ים וְאֶת־הַמִּשְׁפָּטִ֗ים אֲשֶׁ֨ר אָנֹכִ֧י מְצַוְּךָ֛ הַיּ֖וֹם לַעֲשׂוֹתָֽם׃ (פ)
Therefore, observe faithfully the Instruction—the laws and the rules—with which I charge you today.