To comment on a claim, first, open the 'Comments & History' section of the claim. This can be done in multiple ways: left-click on the claim or press the 'C' key while the claim is selected; click on the 
speech bubble icon while hovering on a claim; or, if using a touch-screen device, it may be easiest to click on the three dots that appear at the top-right-hand of a claim while you hover over it, or right-click on the claim, to open the claim menu and then select ‘Comments & History’.
Once you have opened the 'Comments & History' section of a claim, a comment box will appear for you to type your comment. While commenting, you can also click the ‘+’ icon to the left of the comment field to paste a claim from the clipboard, quote a claim, or mention a user. For information on how to ensure that others see your comment, see Mentioning Other Users.
After writing your comment, click the blue arrow to submit your comment, or press the ‘Enter’ key.
To comment on a claim, you need to hold at least Writer-level rights in that discussion (see: Participant Rights and Roles). If you don’t have Writer-level rights or higher, you can still follow the steps above to suggest a comment - for more information, see Suggesting Comments.
This short video will also walk you through the steps required to comment on a claim, as well as several other key features of a Kialo discussion.
For information on how to effectively use comments, see Collaborating With Other Users - Editing, Comments and Marks.