( ( In ( most countries ) ) ( , ( oil ( ( is ( the ( primary ( energy source ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN In) (NP (JJS most) (NNS countries))) (, ,) (NP (NN oil)) (VP (VBZ is) (NP (DT the) (JJ primary) (NN energy) (NN source))) (. .)))
In most countries, oil is the primary energy source.
(ROOT (S (NP (NNP Oil)) (VP (VBZ is) (NP (NP (DT the) (ADJP (RBS most) (RB commonly) (VBN used)) (NN source)) (PP (IN of) (NP (NP (NN energy)) (PP (IN in) (NP (DT the) (NN world))))))) (. .)))