It can be done, he said at last. 
( ( It ( can ( be done ) ) ) ( , ( he ( ( said ( at last ) ) . ) ) ) )
(ROOT (S (S (NP (PRP It)) (VP (MD can) (VP (VB be) (VP (VBN done))))) (, ,) (NP (PRP he)) (VP (VBD said) (PP (IN at) (NP (JJ last)))) (. .)))
It won't be easy, but it can be done. 
( ( ( ( ( It ( ( wo n't ) ( be easy ) ) ) , ) but ) ( it ( can ( be done ) ) ) ) . )
(ROOT (S (S (NP (PRP It)) (VP (MD wo) (RB n't) (VP (VB be) (ADJP (JJ easy))))) (, ,) (CC but) (S (NP (PRP it)) (VP (MD can) (VP (VB be) (VP (VBN done))))) (. .)))