Thomas Jefferson made a metaphor in 1787 when he wrote in a letter, "The tree of liberty must be refreshed from time to time with the blood of patriots and tyrants." He is saying that what happened will not soon be forgotten but we may have to do it again for what we want. ill answer better with choices. :)
Rob is the complete subject since he is the main "character" in the sentence. Mary is secondary. 'Rob' sent a letter to (blank). Without Rob, we wouldn't be reading about sending a letter. Hope that makes sense!