Requesting sense check of C# function for errors and poor practice

         /// 

        ///     Converts input to as close to sentence\ correct case as possible
        /// 

        /// 
        ///     Free text for case correction
        /// 
        /// 
        ///     Free text converted to sentence\ correct case
        /// 
        public string SentenceCase(string input)
        {

            //Break out if there is not work to do
            if (string.IsNullOrEmpty(input))
            {
                return "";
            }

            //Array of characters that signal the next character should be capitalised
            List lineTerminators = new List(new char[] { '.', ':', '\n', '\r', '!', '?' });
            //Indicates if previous character was a lineTerminators
            Boolean start = true;
            //Indicates that currently parsing personal pronoun contraction (e.g. I'll, 'I'd, I'm etc.)
            Boolean personalProNoun = false;
            //outputed string
            char[] output = new char[input.Length];
            //Character previous to current character
            char cPrev = new char();
            //Character following current character
            char cNext;
            //Current character
            char c;

            //Loop through input chars
            for (int i = 0; i < input.Length; i++)
            {

                c = input[i];
                cNext = i < (input.Length - 1) ? input[i + 1] : '.';

                //current character is always lower case if part of personal pronoun contraction
                if (personalProNoun)
                {
                    output[i] = Char.ToLower(c);
                    start = false;
                    //Set personalProNoun to false if the next character is not a letter
                    personalProNoun = Char.IsLetter(cNext);
                }
                //any non-letter characters are added to output
                else if (!Char.IsLetter(c)) 
                {
                    
                    output[i] = c;

                    //Line terminators set start boolean to true
                    if (lineTerminators.Contains(c))
                    {
                        start = true;
                    }
                    //Numbers count as starting a new sentence, other characters do not
                    else if (Char.IsNumber(c) && start)
                    {
                        start = false;
                    }
                }
                //"I" or "i" with no letters either side are assumed to be personal pronoun "I"
                else if ((c.Equals('I') || c.Equals('i')) && !Char.IsLetter(cPrev) && !Char.IsLetter(cNext))
                {
                    //Append "I" in upper case
                    output[i] = Char.ToUpper(c);
                    start = false;

                    //If the following character is apostrophe we are starting a personal pronoun contraction
                    if (cNext.Equals('\''))
                    {
                        personalProNoun = true;
                    }
                }
                //If the last character was a line terminator then current letter is upper case
                else if (start)
                {
                    start = false;
                    output[i] = Char.ToUpper(c);
                }
                //Lower case letters are unchanged
                else if (Char.IsLower(c))
                {
                    output[i] = c;
                    start = false;
                }
                //Only upper case letters remain now
                else
                {
                    start = false;

                    //If this is the start of a se4ntence, add the letter as it is
                    if (start)
                    {
                        output[i] = c;
                    }
                    //if there are uppercase letters either side of this letter, convert to lower case
                    //This allows us to retain the odd upper case character when the freee text is not
                    //solely made up fo upper case letters
                    else if (Char.IsUpper(cPrev) || Char.IsUpper(cNext))
                    {
                        output[i] = Char.ToLower(c);
                    }
                    else
                    {
                        output[i] = c;
                    }
                }

                cPrev = c;

            }

            return new string(output);

        }