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 is impossible. 
( It ( ( is impossible ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (ADJP (JJ impossible))) (. .)))