  In this paper we first prove the so-called large twist theorem, then using it
to prove the boundedness of all solutions and the existence of quasi-periodic
solutions for Duffing's equation $$
\ddot{x}+x^{2n+1}+\dsum_{i=0}^{2n}p_i(t)x^i=0, $$ where $p_i(t)\in
C^1(\mathbb{S}) (n+1\leq i\leq 2n)$ and $p_i(t)\in C^0(\mathbb{S}) (0\leq i\leq
n)$ with $\mathbb{S}=\mathbb{R}/\mathbb{Z}$.
