Another unit was added on to the communal dwelling each time a marriage created a new family.
( ( ( Another unit ) ( was ( ( added ( on ( to ( the ( communal dwelling ) ) ) ) ) ( each time ) ) ) ) ( ( a marriage ) ( ( created ( a ( new family ) ) ) . ) ) )
(ROOT (S (S (NP (DT Another) (NN unit)) (VP (VBD was) (VP (VBN added) (PP (IN on) (PP (TO to) (NP (DT the) (JJ communal) (NN dwelling)))) (NP (DT each) (NN time))))) (NP (DT a) (NN marriage)) (VP (VBD created) (NP (DT a) (JJ new) (NN family))) (. .)))
The only instance of a new unit being added was when a new family is created.
( ( ( The ( only instance ) ) ( of ( ( a ( new unit ) ) ( being added ) ) ) ) ( ( was ( when ( ( a ( new family ) ) ( is created ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (JJ only) (NN instance)) (PP (IN of) (NP (NP (DT a) (JJ new) (NN unit)) (VP (VBG being) (VP (VBN added)))))) (VP (VBD was) (SBAR (WHADVP (WRB when)) (S (NP (DT a) (JJ new) (NN family)) (VP (VBZ is) (VP (VBN created)))))) (. .)))