检查一下是不是form 写成了from
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>表单标签</title>
</head>
<body>
<form action = "#" method = "get">
用户名:<input name="username"><br>
密码:<input name="password"><br>
<input type="submit" value="登录" >
</form>
</body>
</html>