Jesus asks the scribes a question.
"Why do they say ... son?" Jesus uses a question to make the scribes think about who the Messiah is. AT: "Let's think about them saying ... son." or "I will talk about them saying ... son" (See: rc://en/ta/man/translate/figs-rquestion)
The prophets, the religious rulers, and the Jewish people in general knew that the Messiah was the son of David. AT: "everyone says" or "people say" (See: rc://en/ta/man/translate/figs-explicit)
"King David's descendant." The word "son" is used here to refer to a descendant. In this case it refers to the one who would reign over God's kingdom. (See: rc://en/ta/man/translate/figs-synecdoche)
This is a quotation from the book of Psalms which says "Yahweh said to my Lord." But the Jews stopped saying "Yahweh" and often said "Lord" instead. AT: "The Lord God said to my Lord" or "God said to my Lord"
David was referring to the Christ as "my Lord."
The right side is the place of honor. To sit at God’s “right hand” is an idiom that means that Christ will rule as God, with God’s authority. AT: "in the position of God" (See: rc://en/ta/man/translate/figs-idiom)
The Messiah's enemies are spoken of as if they were furniture on which he would rest his feet. This was an image of submission. AT: "until I make your enemies like a footstool for you" or "until I conquer your enemies for you" (See: rc://en/ta/man/translate/figs-metaphor)
In the culture of that time, a father was more respected than a son. David's us of the title 'Lord' for the Christ implies that he was greater than David. (See: rc://en/ta/man/translate/figs-explicit)
"so how can the Christ be David's son?" This can be a statement. AT: "this shows that the Christ is not merely David's descendant" (See: rc://en/ta/man/translate/figs-rquestion)