Main.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Vezba21
{
    public partial class Form1 : Form
    {
        Button[] buttons = new Button[26];
        Button[] brojeviButton = new Button[10];

        public Form1()
        {
            InitializeComponent();
            CreateButtons();
        }

        private void button_Click(object sender, EventArgs e)
        {
            Button button = (Button)sender;
            textBox1.Text += button.Text;
        }
        private void CreateButtons()
        {
            int xLoc = 12;
            int yLoc = 30;
            for (int i = 0; i < 10; i++)
            {

                Button button = new Button();

                button = new System.Windows.Forms.Button();
                button.Location = new System.Drawing.Point(xLoc, yLoc);
                button.Name = "button" + i.ToString();
                button.Size = new System.Drawing.Size(40, 40);
                button.Text = i.ToString();
                button.Click += new System.EventHandler(button_Click);
                Controls.Add(button);
                brojeviButton[i] = button;
                xLoc += 40;
            }
            xLoc = 12;
            yLoc = 70;
            for (char c= 'A'; c <= 'Z';c++)
            {

                Button button = new Button();

                button = new System.Windows.Forms.Button();
                button.Location = new System.Drawing.Point(xLoc , yLoc);
                button.Name = "button" + c.ToString();
                button.Size = new System.Drawing.Size(40, 40);
                button.Text = c.ToString();
                button.Click += new System.EventHandler(button_Click);
                Controls.Add(button);
                buttons[c - 'A'] = button;

                xLoc += 40;

                if (c.ToString() == "M")
                {
                    yLoc = 110;
                    xLoc = 12;
                }

            }
        }

        private void textBox1_KeyUp(object sender, KeyEventArgs e)
        {

            char c = (char)e.KeyValue;
            if (char.IsLetter(c))
            {
                foreach (Button btn in buttons)
                {
                    if (c.ToString() == btn.Text)
                    {
                        btn.BackColor = Color.Transparent;
                    }
                }
            }
            if(char.IsDigit(c))
            {
                foreach(Button btn in brojeviButton)
                {
                    if(c.ToString() == btn.Text)
                    {
                        btn.BackColor = Color.Transparent;
                    }
                }
            }
        }

        private void textBox1_KeyDown(object sender, KeyEventArgs e)
        {
            char c = (char)e.KeyValue;
            if(char.IsLetter(c))
            {
                foreach(Button btn in buttons)
                {
                    if(c.ToString() == btn.Text)
                    {
                        textBox1.Text += c;
                        btn.BackColor = Color.Aquamarine;
                    }
                }
            }
            if (char.IsDigit(c))
            {
                foreach (Button btn in brojeviButton)
                {
                    if (c.ToString() == btn.Text)
                    {
                        textBox1.Text += c;
                        btn.BackColor = Color.Aquamarine;
                    }
                }
            }
        }
    }
}