Ki Tavo: Vaytzav -- An Important Grammatical Form

In parshat Ki Tavo, in the beginning of perek 27, we read:

א וַיְצַו מֹשֶׁה וְזִקְנֵי יִשְׂרָאֵל, אֶת-הָעָם לֵאמֹר: שָׁמֹר, אֶת-כָּל-הַמִּצְוָה, אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם, הַיּוֹם.

1 And Moses and the elders of Israel commanded the people, saying: 'Keep all the commandment which I command you this day.

Even though vaytzav is singular, it is Moshe and the elders who command it. It is just that when you have a list of people doing things, it is acceptable to use the appropriate verb ending for the first person on the list.

We see this also with Miryam, in Bemidbar 12:1:

א וַתְּדַבֵּר מִרְיָם וְאַהֲרֹן בְּמֹשֶׁה, עַל-אֹדוֹת הָאִשָּׁה הַכֻּשִׁית אֲשֶׁר לָקָח: כִּי-אִשָּׁה כֻשִׁית, לָקָח.

1 And Miriam and Aaron spoke against Moses because of the Cushite woman whom he had married; for he had married a Cushite woman.

And we see this also in the beginning of Korach {Bemidbar 16:1}:

א וַיִּקַּח קֹרַח, בֶּן-יִצְהָר בֶּן-קְהָת בֶּן-לֵוִי; וְדָתָן וַאֲבִירָם בְּנֵי אֱלִיאָב, וְאוֹן בֶּן-פֶּלֶת--בְּנֵי רְאוּבֵן.

1 Now Korah, the son of Izhar, the son of Kohath, the son of Levi, with Dathan and Abiram, the sons of Eliab, and On, the son of Peleth, sons of Reuben, took men;

This is obvious, but some online people try to take the וַיִּקַּח in parshat Korach as evidence of multiple authorship, claiming that the other people in the verse obviously had to have been added later, for why else would it have used the singular. This is not necessarily the case.