(1) אם על תורה יקריבנו IF HE OFFER IT FOR A THANKSGIVING — i.e., if he brings it on account of (על) a matter that requires thanksgiving (תודה): on account of a miraculous deliverance that was wrought for him, as being, for instance, one of those who have made a sea-voyage. or travelled in the wilderness, or had been kept in prison, or if he had been sick and was now healed, all of whom are bound to offer thanks-giving, since it is written with reference to them, (Psalms 107:8, 15, 21, 31) “Let them offer thanksgiving to the Lord for His goodness, and for His wonderful works to the children of men!” (
Rabbi Doug Zelden:
There is a beautiful explanation about when the chazzan repeats the Amida aloud and says the Modim prayer and the congregation recites softly a prayer known as 'The Rabbis' Modim.' Why is that? It is because the cantor can recite aloud all the blessings in the Amida and be our agent for such prayers as 'Forgive us,' 'Heal Us,' 'Bless Us with a Good Year,' and so forth. With all our pleas, the cantor or Baal Tfillah can be our public messenger and say the blessing for us, as we answer Amen.
However, there is one thing that no else one can say for us. We must say it for ourselves. That one thing is 'Thank You.' Hoda'ah has to come from ourselves. No one can be our agent to say Thank You.
That is the reason for the 'Rabbis' Modim.'