# coding: utf-8
s = 'Hi He Lied Because Boron Could Not Oxidize Fluorine. New Nations Might Also Sign Peace Security Clause. Arthur King Can.'

target = 1, 5, 6, 7, 8, 9, 15, 16, 19

result = [w[: 1 if i in target else 2] for i, w in enumerate(s.split(), 1)]

print(result)