Commenting on cards
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 "Add comment" textbox in the "Activity" section of the card back.
Add a comment to get the conversation going.
When you comment on a card that you're not already a member of, you'll be automatically Watching that card to help keep up with any replies that come in that don't mention you. If you'd rather not Watch, you can uncheck the "Watch this card for updates" box. After you uncheck the box once, this will default to being off in the future.
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. If you accidentally mention a user who is not a member of the board, they will not receive a notification or gain access of any kind to your board.
You can also mention the card members in a comment by using '@card', and all of the board members by using '@board'.
You can also add emoji to your comment from the smile icon beneath the comment input field or by using colons around your emoji term (ie. :emoji:). Business Class and Trello Gold subscribers can upload custom emoji.
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.
Click "Edit" or "Delete" to edit or delete a comment.
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.