There's a shortcut for nearly everything else, and since some of the shortcuts have changed (a is now m, even though both work, for example) I wonder if there is a shortcut to start a comment on a currently open card.
Trello – Keyboard Shortcut to Start a Comment on a Card
keyboard shortcutstrello
Best Answer
Unfortunately not, though the shortcuts help page will be kept up-to-date as new shortcuts are added.