In the two most famous words of the Shiras Ha'Yam, Moshe and the Jewish people begin their song with "Az yashir," "then [they] will sing." In addition to the many insights offered to explain the word "az," "then," the meaning of the second word, "yashir," or at least its future tense has perplexed many commentaries.
Rashi suggests that after witnessing the splitting of the sea, Moshe and the Jews contemplated beginning to sing, or as Artscroll translates, "Then Moshe and the Children of Israel chose to sing this song...". Rashi then cites a Medrash offering a different explanation. The future tense, "yashir," is indeed referring to a song to be sung in the future, and this pasuk alludes to the revival of the dead. In other words, in addition to the song that the Jews sang on that day, they will sing again another song at a later date, i.e. techiyas ha'meisim.