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It’s always helpful to know the keyboard shortcuts for an app. In Teams, you can press the R key on your keyboard to reply to a conversation. Press Ctrl-O (⌘-O on a Mac) to quickly attach a file to a message. To see all the keyboard shortcuts that are available in Teams, press the Ctrl and period keys together (the ⌘ and period keys on Mac).
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Press Ctrl-. or ⌘-. to see a list of keyboard shortcuts for Teams. (Click image to enlarge it.)