form1：
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace 简单登录界面
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            String name = this.textBox1.Text; // 获取里面的值
            String password = this.textBox2.Text;
            if (name.Equals("admin") && password.Equals("admin")) // 判断账号密码是否等于admin
            {
                MessageBox.Show("登录成功");
            }
            else
            {
                MessageBox.Show("登录失败！");
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            Form2 form2 = new Form2();
            form2.Show();
        }
    }
}

form2：
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace 简单登录界面
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string userName = textBox1.Text;
            string oldPassWord = textBox2.Text;
            string newPassWord = textBox3.Text;
            string checkPassWord = textBox4.Text;                 
            if (oldPassWord != "admin")
            {
                MessageBox.Show("原密码错误！", "提示");
                return;
            }
            if (newPassWord != checkPassWord)
            {
                MessageBox.Show("两次输入的密码不一致！", "提示");
                return;
            }
            if (newPassWord.Length < 7)
            {
                MessageBox.Show("密码长度必须大于6个字符");
                return;
            }
            if (newPassWord == oldPassWord)
            {
                MessageBox.Show("新旧密码不能一样");
                return;
            }
            else
            {
                MessageBox.Show("密码修改成功");
            }    
        }

        private void button2_Click(object sender, EventArgs e)
        {
            Form2 form2 = new Form2();
            form2.Close();
        }
    }
}