Very often the emperor was only a minor, so that the Fujiwara patriarch acted as regent.
( Very ( ( often ( the emperor ) ) ( ( ( ( was ( only ( a minor ) ) ) , ) ( so ( that ( ( the ( Fujiwara patriarch ) ) ( acted ( as regent ) ) ) ) ) ) . ) ) )
(ROOT (S (ADVP (RB Very)) (NP (RB often) (DT the) (NN emperor)) (VP (VBD was) (NP (RB only) (DT a) (JJ minor)) (, ,) (SBAR (RB so) (IN that) (S (NP (DT the) (NNP Fujiwara) (NN patriarch)) (VP (VBD acted) (PP (IN as) (NP (NN regent))))))) (. .)))
Oftentimes, the best emperors were those that were minors. 
( Oftentimes ( , ( ( the ( best emperors ) ) ( ( were ( those ( that ( were minors ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB Oftentimes)) (, ,) (NP (DT the) (JJS best) (NNS emperors)) (VP (VBD were) (NP (NP (DT those)) (SBAR (WHNP (WDT that)) (S (VP (VBD were) (NP (NNS minors))))))) (. .)))