With some exceptions, you need to close the tags that you open. You need the closing </p>, </h1>, </ul>, and </li>, for example.
But you don’t need to close the <img> tag.
A common mistake is to forget the closing </body> and </html> tags in the bottom of the document. (If you always start your HTML document with a template that already closes the body and the html tags, there’s a great chance that you won’t forget to close them.)