Question

why did the framers see the need to have a written constitution, rather than an unwritten one? (such as the british constitution)
your answer

explain what thomas jefferson means, when he says, \the tree of liberty must be refreshed with the blood of patriots and tyrants\
your answer

Explanation:

For the first question: The Framers wanted a written constitution for clarity, to limit government power (as an unwritten one could be more ambiguous and prone to abuse), and to ensure a clear framework for the new nation that was distinct from the British system. A written document could be a more concrete safeguard of rights and structure.
For the second question: Thomas Jefferson's statement means that the preservation and growth of liberty (represented as a tree) often requires sacrifice. Patriots may give their blood (sacrifice their lives) in the fight for liberty, and sometimes the "blood" of tyrants (overthrowing oppressive rulers) is also necessary. It implies that liberty is not a passive state but one that may need active struggle, even violent at times, to maintain and advance.

Answer:

For the first question: The Framers saw the need for a written constitution for clarity, to limit government power, and to have a distinct, concrete framework for the nation.
For the second question: Jefferson means that liberty (like a tree) needs the sacrifice (blood) of those fighting for it (Patriots) and the overthrow of oppressors (Tyrants) to thrive.