sentence: Nowadays , people use fossil fuels as the most important source of energy for travelling and manufacturing . This source of energy does n't always remain at a constant level , and someday it will run out .
modified: Nowaday , people use fossil fuel as the most important source of energy for travelling , manufacturing , this source of energy does n't always maintain at the constant level , but someday it will be run out .
transformation: MORPH, NOUN:NUM, OTHER, PUNCT, VERB, DET, CONJ, VERB:TENSE