Program.cs

using System;
using System.IO;
using System.Text.RegularExpressions;

namespace TextToSubStationAlpha
{
    class Program
    {
        private static readonly Regex onscreenPattern = new Regex("([^\\s]+)", RegexOptions.Compiled);

        static void Main(string[] args)
        {
            // Prepare output file
            string writePath = GetOutputPath();
            // Prepare subtitle file with header information
            string headerPath = GetHeaderPath();
            // Create the subtitle file with the header text
            File.WriteAllLines(writePath, File.ReadAllLines(headerPath));

            // Get the text
            string trans = GetTranslationPath();
            string[] text = File.ReadAllLines(trans);

            // Parse text and create subtitle file
            CreateSubtitles(writePath, text);
        }

        // Prepare output file
        private static string GetOutputPath()
        {
            string writePath;
            string input;

            Console.WriteLine("What folder do you want to write to?");
            while(true)
            {
                input = Console.ReadLine();
                if(Directory.Exists(input))
                    break;
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("That's not a valid path!");
                Console.ResetColor();
            }
            writePath = input;

            Console.WriteLine("What file do you want to write to");
            input = "";
            while(true)
            {
                input = Console.ReadLine();
                if(input.Split('.')[input.Split('.').Length - 1] == "ass" || input.Split('.')[input.Split('.').Length - 1] == "ssa")
                    break;
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("That's not a valid SubStaiton Alpha file extension!\nPlease use \".ass\" or \".ssa\".");
                Console.ResetColor();
                break;
            }

            return writePath + "/" + input;
        }
        // Prepare subtitle file with header information
        private static string GetHeaderPath()
        {
            string input;

            Console.WriteLine("What's the path to your header file? This contains all the subtitle settings and format information.");
            input = "";
            while(true)
            {
                input = Console.ReadLine();
                if(File.Exists(input))
                    break;
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("That file doesn't exist!");
                Console.ResetColor();
            }

            return input;
        }
        // Get path to translation file
        private static string GetTranslationPath()
        {
            string input;

            Console.WriteLine("What file do you want use as the translation file?");
            input = "";
            while(true)
            {
                input = Console.ReadLine();
                if(File.Exists(input))
                    break;
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("That file doesn't exist!");
                Console.ResetColor();
            }

            return input;
        }
        // Parse text and create subtitle file
        private static void CreateSubtitles(string writePath, string[] text)
        {
            string endTime = "09:27";
            string[] current = null;
            string[] next = null;
            bool alt = false;
            string time1 = "";
            string time2 = "";
            string line1 = "";
            string line2 = "";

            for (int i = 0; i < text.Length; i++)
            {
                //Get elements
                current = text[i].Split('\t');
                if (i != text.Length - 1)
                    next = text[i + 1].Split('\t');

                //Check if text has modifiers
                if (current[0].Contains(' '))
                    alt = true;

                //Get the raw start time
                time1 = onscreenPattern.Match(current[0]).Captures[0].Value;
                if (next != null)
                {
                    //Get raw end time
                    time2 = onscreenPattern.Match(next[0]).Captures[0].Value;

                    //Insert times and text
                    if (alt)
                        line1 = $"Dialogue: 0,0:{time1}.00,0:{time2}.00,Alt Dialogue,,0,0,0,,{{\\be2}}{current[1]}";
                    else
                        line1 = $"Dialogue: 0,0:{time1}.00,0:{time2}.00,Default,,0,0,0,,{{\\be2}}{current[1]}";
                    if (current[2] != "")
                        line2 = $"Dialogue: 0,0:{time1}.00,0:{time2}.00,Notes,,0,0,0,,{{\\be2}}{current[2]}";
                }
                else
                {
                    //Insert times and text
                    if (alt)
                        line1 = $"Dialogue: 0,0:{time1}.00,0:{endTime}.00,Alt Dialogue,,0,0,0,,{{\\be2}}{current[1]}";
                    else
                        line1 = $"Dialogue: 0,0:{time1}.00,0:{endTime}.00,Default,,0,0,0,,{{\\be2}}{current[1]}";
                    if (current[2] != "")
                        line2 = $"Dialogue: 0,0:{time1}.00,0:{endTime}.00,Notes,,0,0,0,,{{\\be2}}{current[2]}";
                }

                //Write lines to ouput file
                if (line2 != "")
                    line1 = "\n" + line1 + "\n" + line2;
                else
                    line1 = "\n" + line1;
                File.AppendAllTextAsync(writePath, line1);

                //Reset current values
                line1 = "";
                line2 = "";
                alt = false;
                current = null;
                next = null;
                time1 = "";
                time2 = "";
            }
        }
    }
}