"Vayir'u ha'am es Hashem...", the awe of G-d captivated Bnei Yisrael on the banks of Yam Suf and gave rise to "Az yashir", the desire and thought to sing shirah in the future (hence, as Rashi explains, "yashir" is in the future tense). The Berdichiver elaborates on this transition between the past and future tenses in the two verses. Encountering G-dliness is overwhelming; it leaves a person literally at loss for words. It is only after the encounter has ended, after the experience has passed, that a person can find the means to give voice to the emotions that were part of that moment. The splitting of the sea was a moment of overwhelming yiras shamayim during which speech was impossible. "Az yashir" -- the song of praise to G-d arose in thought, but could only be voiced in the future after that awesome moment had passed.
Isru chag of Pesach. Sometimes we become so entrapped by the experiences of the Chag in every sense, be it the seder or the chol hamoed outing, that we cannot really formulate or articulate what those experiences mean. It is only after the chag, after that encounter with ruchniyus has passed, sometimes long after, that we can pause to reflect and give voice to some of the meaning those moments had. Isru chag starts the transition from "vayir'u ha'am es Hashem" to the song of "Az yashir".