Fisrt try

CommandParser.cs

@@ -0,0 +1,64 @@
+using System.Text.Json;
+namespace SectTask;
+
+public class CommandParser
+{
+    public void Commands(string?[] args)
+    {
+        Methods td = new Methods();
+        for (int i = 0; i < args.Length - 1; ++i)
+        {
+            try
+            {
+                switch (args[i])
+                {
+                    case "/add":
+                        SubTask sub = new SubTask(Convert.ToBoolean(args[i + 1]), args[i + 2], args [i + 3]);
+                        td.AddTask(Convert.ToInt32(args[i + 4]), sub,Convert.ToDateTime(args[i + 5]),Convert.ToBoolean(args[i + 6]));
+                        break;
+                    case "/all":
+                        td.ShowTasks();
+                        break;
+                    case "/delete":
+                        td.DeleteTask(Convert.ToInt32(args[i + 1]));
+                        break;
+                    case "/save":
+                        td.toFile(args[i + 1]);
+                        break;
+                    case "/load":
+                        td.fromFile(args[i + 1]);
+                        break;
+                    case "/completed":
+                        td.ChangeStatus(Convert.ToInt32(args[i + 1]));
+                        break;
+                    case "/create-group":
+                        td.CreateGroup(Convert.ToInt32(args[i + 1]));
+                        break;
+                    case "/delete-group":
+                        td.DeleteGroup(Convert.ToInt32(args[i + 1]));
+                        break;
+                    case "/add-to-group":
+                        td.AddToGroup(Convert.ToInt32(args[i + 1]), Convert.ToInt32(args[i + 2]));
+                        break;
+                    case "/delete-from-group":
+                        td.DeleteFromGroup(Convert.ToInt32(args[i + 1]), Convert.ToInt32(args[i + 2]));
+                        break;
+                    case "/add-subtask":
+                        SubTask sub1 = new SubTask(Convert.ToBoolean(args[i + 1]), args[i + 2], args [i + 3]);
+                        td.AddSubTask(Convert.ToInt32(args[i + 2]), sub1 );
+                        break;
+                    case "/change-sub-status":
+                        td.ChangeSubStatus(Convert.ToInt32(args[i + 1]));
+                        break;
+                    case "/today":
+                        td.DayCheck();
+                        break;
+                }
+            }
+            catch (ArgumentException ep)
+            {
+                Console.WriteLine(ep.Message);
+            }
+        }
+    }
+}

Methods.cs

@@ -0,0 +1,197 @@
+namespace SectTask;
+using System.Text.Json;
+
+public class Methods
+{
+    private List<Task?> _taskData = new List<Task?>();
+    private List<Group?> _groupData = new List<Group?>();
+    private List<SubTask?> _subData = new List<SubTask?>();
+
+    public void AddTask(int id, SubTask sub, DateTime deadline, bool complete)
+    {
+        _taskData.Add(new Task() {TaskId = id, Sub = sub, DeadLine = deadline, TaskStatus = complete} );
+        Console.WriteLine("Task Created!");
+    }
+
+    public void AddSubTask(int Taskid, SubTask sub)
+    {
+        foreach (var t in _taskData.Where(t => t!.TaskId == Taskid))
+        {
+            t!.Sub = sub;
+        }
+
+        Console.WriteLine("SubTask Created!");
+    }
+
+    public void ChangeSubStatus(int id)
+    {
+        foreach (var t in _subData.Where(t => t!.SubStatus == false))
+        {
+            t!.SubStatus = true;
+        }
+        Console.WriteLine("Status Changed!");
+    }
+    public void ShowTasks()
+    {
+        foreach (var t in _taskData)
+        {
+            Console.WriteLine(t!.TaskId);
+            Console.WriteLine(t!.TaskStatus);
+            var json = JsonSerializer.Serialize(t?.Sub);
+            var json1 = JsonSerializer.Serialize(t?.DeadLine);
+            Console.WriteLine(json);
+            Console.WriteLine(json1);
+        }
+    }
+
+    public void ShowTasks(int id)
+    {
+        foreach (var t in _taskData)
+        {
+            if (t!.TaskId == id){
+                Console.WriteLine(t!.TaskId);
+                Console.WriteLine(t!.TaskStatus);
+                var json = JsonSerializer.Serialize(t?.Sub);
+                var json1 = JsonSerializer.Serialize(t?.DeadLine);
+                Console.WriteLine(json);
+                Console.WriteLine(json1);
+            }
+            else
+            {
+                Console.WriteLine("Wrong Id");
+            }
+        }
+    }
+
+    public void DeleteTask(int id)
+    {
+        for (var i = 0; i < _taskData.Count; ++i)
+        {
+            if (_taskData[i]!.TaskId == id)
+            {
+                _taskData.Remove(_taskData[i]);
+            }
+        }
+    }
+
+    public void ChangeStatus(int id)
+    {
+        foreach (var t in _taskData.Where(t => t!.TaskStatus == false))
+        {
+            t!.TaskStatus = true;
+        }
+        Console.WriteLine("Status Changed!");
+    }
+
+    public void toFile(string? ph)
+    {
+        var str = new StreamWriter(ph!);
+        foreach (var t in _taskData)
+        {
+            str.WriteLine(t!.TaskId);
+            str.WriteLine(t!.TaskStatus);
+            var json = JsonSerializer.Serialize(t?.Sub);
+            var json1 = JsonSerializer.Serialize(t?.DeadLine);
+            str.WriteLine(json);
+            str.WriteLine(json1);
+        }
+        str.Close();
+    }
+
+    public void fromFile(string? ph)
+    {
+        if (File.Exists(ph))
+        {
+            using StreamReader reader = new StreamReader(ph);
+            _taskData.Clear();
+            for (int i = 0; i < System.IO.File.ReadAllLines(ph).Length; ++i)
+            {
+                string? json = reader.ReadLine();
+                if (json != null) _taskData.Add(JsonSerializer.Deserialize<Task>(json));
+            }
+        }
+        else
+        {
+            Console.WriteLine("File not found");
+        }
+    }
+
+    public void DayCheck()
+    {
+        foreach (var json in from t in _taskData where t!.DeadLine.Day == DateTime.Now.Day select JsonSerializer.Serialize(t))
+        {
+            Console.WriteLine(json);
+        }
+    }
+
+    public void CreateGroup(int id)
+    {
+        _groupData.Add(new Group(){GroupId = id,Tasks = null});
+    }
+
+    public void AddToGroup(int Taskid, int Groupid)
+    {
+        foreach (var t in _taskData.Where(t => t!.TaskId == Taskid))
+        {
+            foreach (var t1 in _groupData.Where(t1 => t1!.GroupId == Groupid))
+            {
+                t1!.Tasks = t;
+            }
+        }
+    }
+
+    public void ShowGroup()
+    {
+        foreach (var t in _groupData)
+        {
+            var json = JsonSerializer.Serialize(t!.GroupId);
+            var json1 = JsonSerializer.Serialize(t!.Tasks);
+            if (json == "null")
+            {
+                Console.Write("");
+            }
+            else
+            {
+                Console.WriteLine(json);    
+            }
+            if (json1 == "null")
+            {
+                Console.Write("");
+            }
+            else
+            {
+                Console.WriteLine(json1);    
+            }
+        }
+    }
+
+    public void DeleteGroup(int id)
+    {
+        foreach (var t in _groupData)
+        {
+            if (t!.GroupId == id)
+            {
+                t.Tasks = null;
+                t.GroupId = null;
+            }
+        }
+    }
+
+    public void DeleteFromGroup(int Groupid, int Taskid)
+    {
+        for (int i = 0; i < _groupData.Count; ++i)
+        {
+            if (_groupData[i]!.GroupId == Groupid)
+            {
+                for (int j = 0; j < _taskData.Count; ++j)
+                {
+                    if (_taskData[j]!.TaskId == Taskid)
+                    {
+                        _groupData[i]!.Tasks = null;
+                    }
+                }
+            }
+        } 
+    }
+
+}

Program.cs

@@ -0,0 +1,12 @@
+using System.ComponentModel.Design.Serialization;
+
+namespace SectTask;
+
+static class Program
+{
+    static void Main(string?[] args)
+    {
+        CommandParser input = new CommandParser();
+        input.Commands(args);
+    }
+}

Task.cs

@@ -0,0 +1,43 @@
+namespace SectTask;
+
+public class SubTask
+{
+    public bool SubStatus { get; set; }
+
+    public string? Name { get; set; }
+
+    public string? Description { get; set; }
+
+    public SubTask(bool complete, string? name, string? description)
+    {
+        SubStatus = complete;
+        Name = name;
+        Description = description;
+    }
+
+}
+
+public class Task
+{
+    public SubTask Sub;
+    public int TaskId { get; set; }
+
+    public bool TaskStatus { get; set; }
+    public DateTime DeadLine { get; set; } // 01.01.0001 0:00:00
+
+    public Task(int id, SubTask sub, DateTime deadline, bool status)
+    {
+        TaskId = id;
+        Sub = sub;
+        DeadLine = deadline;
+        TaskStatus = status;
+    }
+
+    public Task() { }
+}
+
+public class Group
+{
+    public Task? Tasks;
+    public int? GroupId { get; set; }
+}

sectreborn.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <OutputType>Exe</OutputType>
+        <TargetFramework>net6.0</TargetFramework>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+    </PropertyGroup>
+
+</Project>