filename = input("Enter filename: ")

with open(filename, "r") as f:
    the_file = f
    with open("sentences.txt", "w+") as w:
        string_of_words = ""
        for word in the_file:
            word = word.strip()
            if word == ".":
                string_of_words += "."
                print(string_of_words)
                print(string_of_words, file=w)
                string_of_words = ""
            else:
                if string_of_words != "":
                    if word == ",":
                        string_of_words += ','
                    else:
                        string_of_words += " " + word
                else:
                    string_of_words += word