Rabbi Roodyn

Rav Hirsch develops this idea by saying that any song that stems from this contemplation of this state of the future is called a שיר החדש. This is based on the Pasuk in Tehillin 33:3 which says שירו לו שיר החדש היטיבו נגן בתרועה , which he translates as, ‘Sing to Him a new song, express it in tones as is seemly, with deep inner emotion.’

He explains that the word שיר  is primarily used to denote ‘singing praises’, proclaiming the show of Hashem’s Hand in the course of history. The word שירה sings of the invisible aspect of Divine rule in the events of the past and present, but  שיר refers to those signs of His sovereignty that will become apparent only some day in the future.

שיר and particularly  שיר חדשare always employed with reference to the eventual redemption of the world, to which all the developments of history will ultimately lead.

This is based on a Medrash  (Mechilta Beshalach) that says that the female (who  has the gift of being able to nurture and give birth to future life) hints at a redemption that will have future redemptions after it. Males on the other hand do not give birth and as such the masculine form  שיר relates to a redemption that is final and has no salvation after it.

Rav Hirsch therefore says that the masculine form  שיר refers to that salvation which will be  the final splendid goal of the entire march of time, rather than a redemption that is a stepping stone to another, future redemption.