Why is Roger unable to say thank you to Mrs. Jones in "Thank You, M'am"?
Your original question asked two questions and, according to enotes regulations, I have edited it down to one question. Please do not ask multiple questions.
To consider the question above, how do we respond to acts of random and unsought-for sacrificial kindness, as characterised by Mrs. Jones? One of the qualities that makes this story so powerful and memorable is that Mrs. Jones acts completely differently from how we would expect her to act. There aren't many people that would stop a thief and then take them home to share food with them, taking time to find out about their lives. It is surprise and shock that is expressed by Roger throughout the tale as he is dragged along and passively sits with Mrs. Jones, then said goodbye too before he is able to formulate a response or a way of saying thank you for such an unexpected act of kindness:
They boy wanted to say something other than "Thank you, m'am" to Mrs. Luella Bates Washington Jones, but although his lips moved, he couldn't even say that as he turned at the foot of the barren stoop and looked up at the large woman in the door. Then she shut the door.
It appears that, for some acts of kindness, there are no words to express how we feel and the gratitude that overwhelms us. The kindness of Mrs. Jones literally leaves Roger speechless, and he is not even able to stammer out the "Thank you" that he knows is insufficient.
Roger wasn’t used to being cared for and taught right from wrong before. The fact that the woman which he had previously tried to rob had taken him home and fed him then gave him money was enough to leave him shocked. He was so overwhelmed by feelings of gratitude and thankfulness towards this kind lady that he was left speechless. Words could never repay her for what she had just done; changing his life with nothing but a small act of kindness.