Beshalach is Singing

Then…sang: Heb. אָז יָשִׁיר. [The future tense presents a problem. Therefore, Rashi explains:] Then, when he [Moses] saw the miracle, it occurred to him to recite a song, and similarly, “Then Joshua spoke (אָז יְדַבֵּר יְהוֹשֻׁעַ)” (Josh. 10:12); and similarly, “and the house [which] he would make (יַעֲשֶׂה) for Pharaoh’s daughter” (I Kings 7: 8), [which means] he decided to make it for her. Here too, יָשִׁיר [in the future tense means that] his heart dictated to him that he should sing, and so he did, “and they spoke, saying, I will sing to the Lord.’ ” Likewise, with [the above reference to] Joshua, when he saw the miracle [of the defeat of the Amorite kings (Josh. 10:11)], his heart dictated to him that he speak [praises to God], and so he did, “and he said in the sight of Israel” (Josh. 10:12). Likewise, the song of the well, with which [Scripture] commences: “Then Israel sang (אָז יָשִׁיר)” (Num. 21:17), it explains after it, “Ascend, O well!, sing to it.” [I.e., in these three instances, the “yud” of the future tense denotes the thought, and after each one, Scripture continues that the thought was brought to fruition.] “Then did Solomon build (אָז יִבְנֶה) a high place” (I Kings 11:7); the Sages of Israel explain that he sought to build [it] but did not build [it] (Sanh. 91b). We [thus] learn that the “yud” may serve to indicate a thought. This is to explain its simple meaning, but the midrashic interpretation is [as follows]: Our Rabbis of blessed memory stated: From here is an allusion from the Torah to the resurrection of the dead (Sanh. 91b, Mechilta), and so it is [i.e., the future tense is used] with them all, except that of Solomon, which they explained as [implying] “he sought to build but did not build.” One cannot say and explain this form like other words written in the future, but which mean [that they occurred] immediately, such as “So would Job do (וָעִשֶׂה)” (Job 1:5); “by the command of the Lord would they encamp (יַחֲנוּ)” (Num. 9:23); “And sometimes the cloud would be (יִהְיֶה)” (Num. 9:21),

because that is [an example of] something that occurs continually, and either the future or the past is appropriate for it, but that which occurred only once [i.e., the song that was sung], cannot be explained in this manner. —

Az Yashir. The psauk didn't have to say "az" rather "vayashar" (And Moshe sang..) And the idea is understood because they really sang. However, the pasuk is intended to make us aware of another meaning. That when the great fear and complete belief in God entered into their hearts, then they merited to sing this with divine inspiration "ruach ha kodesh". And the pasuk says Yashir, which is future tense, that a person might be mistaken and say that the kind of song they sang was totally unique and rare and could only be sung then. The Az is there to teach us that we will merit to sing it also in the future, and that all who come to sing the song befure Hashem, will be able to. OR to hint to us the mitzvah 0 that there is a mitzvah to say this (section) always, and it is established in shacharit prayer every day.