We are known as the Am Hashem for a reason. More so than any other nation in the world, each member of Klal Yisroel has a unique ability to form a strong relationship with Hashem. This relationship starts at the very beginning of our day.
The Mekuballim who lived in Tzfas in the 1500’s developed some of the Tefilos that enhance our Dveikus Bashem. One such person was Rav Moshe Ben Reb Yehudah ibn Machir, who, it seems composed the Modeh Ani. He wrote a Sefer called “Seder HaYom” which is cited by the Achronim on Shulchan Aruch. It was printed in Venice, Italy in 1599. The Mogain Avrohom and the Eliyahu Rabbah both cite it as something that must be said.
The Mishna Brurah quotes it and writes, “When we arise in the morning we say, ‘Modeh Ani lefanecha, melech chai v’kayam, shehechezarta bi nishmasi b’chemla, rabah emunasecha.’ This means, ‘Before You, I thank you, O’ living and everlasting King, that You have returned my soul to me in mercy.. How great is your faithfulness!’
Why do we say this? The reason is that our bodies are not just hydro-carbon based cells, proteins, water and minerals. We are also imbued with a Neshama, a soul, – called in Sefer Iyov a “Chailek Elokah Mimaal. – A Divine portion from Above.” When we sleep at night, our Neshamos leave our physical bodies and arise to Heaven. Hashem in His Chemla – kindness, returns our Neshama to us when we wake up in the morning. The Modeh Ani is a way of expressing gratitude which makes us into better people, but it is also more than this. There is an element of yearning for Geulah in it too.
The last words, “Great is your faithfulness is based upon a verse in Aicha (3:23), “They are new every morning, great is Your faithfulness.” The Psikta Zutrasa explains that since each and every morning You renew our souls, we know that in the future You will redeem us as well.
The words “that You have returned my soul to me” come from the Avudraham, a Spanish Rishon, in his Hilchos Krias Shma. Yet, he did not advocate this formula as something to recite each morning upon awakening. It seems that the author of the Seder HaYom combined these elements together in conceiving of the Modeh Ani.
However it happened, the Modeh Ani entered into the ouevre of Jewish Tefilos, much in the same manner that Kabbalas Shabbos did.
A woman does not change the wording to Modah instead of Modeh. Rav Dovid Feinstein Shlita explains, “This is the nusach that was established, and we do not change it.”