Add a checklist, comments and files

Break a card into steps, discuss it, mention a teammate and attach files.

In short

A checklist breaks a card into steps you tick off. Comments are where the team discusses the card, and an @mention brings someone in. Files attached to a card can be downloaded by anyone who can open the board.

Before you start

Who: checklist steps and files need "Change cards". Comments need Comment access or more, so a share at View cannot comment. Anyone who can open the board reads and downloads.
Plan: every plan. A file counts toward the plan's storage and must fit its largest-file limit (see /help/settings/storage-limits).
First: open a card on a board you can open.

Steps

Open the card and its "Sub-items" tab. Under "Checklist", type a step and press Enter.
Tick a step to complete it. The tab shows progress, such as 3 of 5.
To give a step to someone, press the pencil on the step and choose them in the list beside the due date, then save. They start watching the card and are told the step is theirs; their name shows on the step.
When a step grows into work of its own, press the arrow on the step ("Make … a child card"). It becomes a child card of this card with the step's text as its title and the step's owner and due day, and leaves the checklist. Guests of the board cannot do this.

In the "Activity" tab, under "Comments", write an update.
Type @ and the start of a name, pick a person with the arrow keys and Enter, or click it.
Press Ctrl+Enter to post.

Under a comment, choose "Reply" to answer in its thread. A reply to a reply joins the same thread.
Choose + under a comment and pick a reaction. Choose your reaction again to take it back.
When the discussion is settled, choose "Resolve" under the thread. It folds to one line; "Show thread" opens it and "Reopen" opens it again.

In the "Related" tab, under "Files", choose "Add files", or drop files on the tab.
A picture shows as a small image. Press the expand button beside a picture or a text file to look at it without downloading it.
When the very same file is on other cards you can open, "Also on" names them under the file; press one to open that card.

Result: the person you mentioned is notified, starts watching the card, and sees it on Home under Needs you until they reply.

Good to know

Each checklist step can carry its own due date, separate from the card's.
Edit your own comment from its … menu; it then says edited. Its author or a board manager can delete it. History records that a comment was added, edited or removed, never what it said.
The mention list offers the people who can see the board. A comment can mention a limited number of people.
Anyone who can open the board can download files. Whoever added a file, or a board manager, can remove it, which deletes it for everyone for good.
The largest file depends on your plan: 10 MB on Free, up to 100 MB on Pro and Business. Files count toward the plan's storage.
What happens next: a mentioned person is notified, starts watching and sees the comment under Needs you on Home until they reply. A file you remove is deleted for everyone for good.
To undo: untick a step to reopen it. A comment's author can edit or delete it.
A reply in a resolved thread opens it again. Reactions notify nobody and leave no line in the history; resolving and reopening a thread do leave one.
When someone mentions you or answers your comment, you can also get an email with the comment. Reply to that email from your own address and your words land in the thread as a comment, without opening {{brand}}. A card sends you at most one such email in ten minutes, and the link in the email stops them.
Anyone who may comment can reply, react and resolve, a guest with "Can comment" too. Someone with View reads the threads and the reactions.
A guest with "Can comment" can comment too (see /help/boards/board-guests).

If something goes wrong

"An attachment can be at most" a size: the file is larger than your plan allows.
"Only its author can edit a comment." and "Only its author or a board manager can remove a comment.": ask the author or a manager.
"You can view this board but not comment on its cards.": your access is View. Ask for Comment.
"A comment can mention at most 20 people.": mention fewer people.
"A reply goes under a comment on the same card.": the comment you answered was removed or is on another card. Write a new comment instead.
"A comment needs text of 4000 characters or fewer." and "A checklist item needs text of 300 characters or fewer.": shorten the text.