How to password protect posts in WordPress
In the admin panel of your WordPress there are options with which you can protect with passwords any of your posts/articles. You can do this from the settings of the particular post.
After you log in to the admin panel click on the Posts menu on the left. On the page that opens, depending on whether you want to add a new post or edit an existing one, click on the Add New button at the top of the page or on the Edit button of an existing post. On the add/edit form with the post settings click on the Edit button for the Visibility setting in the Publish block. A few radio buttons will appear. Mark the radio button Password protected and type a password of your choice in the text field that appears just under the radio button. Then click on the Update button (or the Publish button if it's a new post). The button is in the Publish block too.
The post will be shown to all users who visit your site but only the title will be visible. In order for the user to be able to view the content of the post they have to type the password in the text field that's provided just under the title of the post.