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))))))) (. .)))
It was not possible to be an emperor unless you were at least 18. 
( It ( ( ( was not ) ( possible ( to ( ( be ( an emperor ) ) ( unless ( you ( were ( ( at least ) 18 ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBD was) (RB not) (ADJP (JJ possible) (S (VP (TO to) (VP (VB be) (NP (DT an) (NN emperor)) (SBAR (IN unless) (S (NP (PRP you)) (VP (VBD were) (NP (QP (IN at) (JJS least) (CD 18))))))))))) (. .)))