Comments allow you to engage in a conversation on the back of a card. To view the back of a card, click on the card to open it.
You can add comments to a card via the "Write a comment" text box in the "Activity" section of the card back.
When you comment on a card that you're not already a member of, you can Watch that card to help keep up with any replies that come in that don't mention you. To do so, you’ll need to check the “Watch” box.
Add an attachment
Add an attachment to the activity feed while making comments on a card by clicking the paperclip icon and selecting the file that you would like to attach to your card.You can also use this to attach a card or board from Trello.
You can mention people in comments by using "@" followed by the user's username or by clicking the @ icon below the comment input field then selecting the members you would like to mention. The mentioned user(s) will get a notification about the comment.
Note: you can mention user’s that are members of your Workspace on any of your boards, as well. If the tagged user is not a member of the board, they will be notified if the visibility of that board is set to “Workspace visible”, but they won’t receive notifications if the visibility is set to “Private”.
You can also mention the card members in a comment by using '@card', and all of the board members by using '@board'.
Editing & Deleting
Edit or delete comments by clicking the "edit" or "delete" links beneath the comment. You can edit or delete any comment you make on any board. Although it's not possible to edit comments made by another user, board admins can delete comments made by any non-admins on their boards. Deleting a comment is permanent.
Sharing a link to a comment
To get a link to a specific comment, right-click the timestamp link under the comment and select your browsers "Copy Link" option.
Working with others
If another user is typing a comment at the same time as you, you'll see their avatar above the comment box, with a speech bubble.