Lista_A = [['de','do','da','ou'],['ae','ay','yhh','oo'],['ow','pa','la','for']]
Lista_A_new = []
for lista in Lista_A:
Lista_A_new.extend(lista)
print(Lista_A_new) #['de', 'do', 'da', 'ou', 'ae', 'ay', 'yhh', 'oo', 'ow', 'pa', 'la', 'for']
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['de', 'do', 'da', 'ou', 'ae', 'ay', 'yhh', 'oo', 'ow', 'pa', 'la', 'for']