Moshe was talking to a crowd. Why am I saying his line to myself before I pray?
In the Torah, the "I" is Moshe, and he is speaking to all Israel. The siddur borrows his sentence for the moment before the Amidah. So it is fair to ask what he meant, and why it fits here.
Rashi reads it as a cue. "Ki" means "when," so Moshe is saying: when I call out God's Name, you give greatness to God. The Gemara in Yoma has the same reading. Rabbi Yehuda HaNasi (the sage who edited the Mishnah) taught that Moshe told the people: when I mention God's Name, you give greatness.
Sforno (an Italian Torah commentator of the 1500s) reads "call the Name" differently. He says one who calls God's Name is one who prays, and he gives two proofs. In Eichah, "I called Your Name, Hashem, from the lowest pit." In Tehillim, Shmuel is "among those who call His Name," and "they call to Hashem and He answers them." On his reading, Moshe is about to pray inside this song, and he asks the people, who know God's greatness from the Torah, to give greatness to God.
Ibn Ezra (a Spanish commentator of the 1100s) reads "give greatness" as aimed at the heavens and the earth, the two that Moshe called to listen in the first verse.
So the commentators do not agree on who is being addressed. And we did not find a source that says why this verse was picked for the spot before the Amidah. If you take Sforno's reading, though, the fit is easy to see: the one who calls God's Name is the one who prays, and that is you, about to begin. That connection is ours, not Sforno's.
Rashi on Devarim 32:3,Yoma 37a,Sforno on Devarim 32:3,Ibn Ezra on Devarim 32:3,Eichah 3:55,Tehillim 99:6Why does the Gemara single out Mincha? Isn't every prayer important?
The Gemara says: Rabbi Chelbo said in the name of Rav Huna that a person should always be careful with Mincha, because Eliyahu was answered only at Mincha. The Gemara goes on to say that other sages make the same point about the other two prayers. Rabbi Yochanan says it of Maariv, from a verse in Tehillim. Rav Nachman bar Yitzchak says it of Shacharit. So Mincha is not the only one with a proof.
Why start with Mincha, then? The Tur gives a practical reason. Shacharit has a clear moment, right after you get out of bed, before you are busy with your affairs. Maariv has one too, when you come home and are free. But Mincha falls in the middle of the day, while you are still tied up with your work. So, the Tur says, you have to take it to heart, clear away your business, and pray. He adds that if you do, your reward is great.
Peninei Halakhah (a modern guide to Jewish law by Rabbi Eliezer Melamed) agrees. It says Mincha is the one a person is often too busy for, and that is why a person must overcome the pressure and set aside time.
It then offers a further thought, and it says "perhaps." The word minchah also means a gift. At Shacharit we have already thanked God for what He gave us. Mincha is a gift that rises out of our own daily life and work.
So the Tur's reason is about the shape of the day: Mincha is the prayer easiest to lose.
Berakhot 6b,Tur, Orach Chaim 232,Peninei Halakhah, Prayer 24:1The song says Israel will turn away and suffer. Why open it with "give greatness to God"?
This is not a gentle song. In Devarim 31, God tells Moshe to write it and teach it so it will be a witness against the people. It will speak up when many troubles come, and it will not be forgotten, because their children will have it in their mouths.
Then Moshe begins. First he calls on heaven and earth to listen. Then he asks that his words fall like rain. Then comes our verse: when I call the Name of Hashem, give greatness to our God. And the next verse says: "The Rock, His work is perfect, for all His ways are just."
Sforno takes the question head on. He explains "give greatness" this way: do not ascribe change to God because of what the song will say about Israel's future, and that He would turn into an enemy to you. In other words, when you hear the hard parts, do not decide that God has changed. (In the text of Sforno we have, the comment breaks off in mid-sentence, so this is all we can quote of it.)
At the end of the song, Moshe adds his own note to the people. Take these words to heart, he says, because they are not an empty thing for you. They are your life.
So the song opens by asking for greatness, and it closes by calling itself life. What is in between is hard.
Devarim 31:19-21,Devarim 32:1-4,Sforno on Devarim 32:3,Devarim 32:44-47