Deuteronomy 31:19

"...this song." In reference to [Deut. 32:1] "Listen, you heavens, and I will speak" through [Deut 32:43] "...and make atonement for his land and people."

And now, write you this hymn, and teach the children of Israel; put it upon their lips, that this hymn may be before them, for a witness against the children of Israel.

write yourselves [plural] Not only Moshe, but also everyone who knows how to write, is commanded to do so [Sanhedrin 21b].

teach it i.e., he is to explain the difficult meanings to them.

place it into their mouths that they should know it word for word.

The word teach it (as well as the word place it) is unusual, because according to the rule, the heh ought to be vocalized with a qamaṣ , and the daled should be vocalized with a segol....

a witness concerning the children of Israel Either This is meant to be joined to the subsequent phrase I shall bring them — that is, the poem is a witness that I shall bring them. or, more plausibly, it is to be contrasted to that passage, on account of the subsequent will respond ([Hebrew: veantah] as in, “do not speak up [Hebrew: taeneh] against your neighbor as a false witness” [Exodus 20: 13]). In other words: The poem will answer those who will say, “Why has all this befallen us?”

AND NOW WRITE YE THIS SONG. In line with the plain meaning of Scripture, He commands both Moses and Joshua to write it for He wanted to make Joshua His prophet during Moses’ lifetime. Now, Moses wrote it while Joshua stood by him, read it, and saw [it completed], this being the sense of the verse, So Moses wrote this Song the same day, and taught it the children of Israel. It refers to Moses who was the principal [in the writing of the Song] for it was from him that they would hear it and learn it, although Joshua, too, taught it with him, as it is stated at the conclusion [of the chapter] he [i.e., Moses], and Hoshea, the son of Nun.
Now, the meaning of the expression this Song is “the Song that I will now tell you,” this being ‘Ha’azinu’ (Give ear)., Verses 1-43. It is called Song because Israel would recite it with song and psalm. was chanted each Sabbath during the Service of the Additional Offering. The cycle of the Song was completed every six weeks. Therefore, the six divisions (Aliyoth) in the reading of this Song in the Synagogue on the Sabbath of Ha’azinu, follow the same divisions as sung in the Sanctuary (Rosh Hashanah 31a). It was also written [in the Torah] in the style of a song, since songs are written with interruptions [empty spaces] indicating the places of chanting.

השירה, the entire book of Deuteronomy is called שירה, “Song, Poem.”

הזאת, the portion Haazinu, which testifies concerning the impending disasters if the people were to abandon the path of Torah. If and when this would come to pass, they would know exactly to what to ascribe their misfortune.