1.Command-X: Cut the selected item and copy it to the Clipboard.
2.Command-C: Copy the selected item to the Clipboard. This also works for files in the Finder.
3.Command-V: Paste the contents of the Clipboard into the current document or app. This also works for files in the Finder.
4.Command-Z: Undo the previous command. You can then press Shift-Command-Z to Redo, reversing the undo command. In some apps, you can undo and redo multiple commands.
5.Command-A: Select All items.
6.Command-F: Find items in a document or open a Find window.
7.Command-O: Open the selected item, or open a dialogue to select a file to open.
8.Command-P: Print the current document.
9.Command-S: Save the current document.
10.Command-T: Open a new tab.
11.Command-W: Close the front window.
To close all windows of the app,
press Option-Command-W.
12.Option-Command-Esc: Force quit an app.
13.Control-Command-F: Use the app in full screen, if supported by the app.
14.Space bar: Use Quick Look to preview the selected item.
15.Command-Tab: Switch to the next most recently used app among your open apps.
16.Shift-Command-5: In macOS Mojave or later, take a screenshot or make a screen recording. Or use Shift-Command-3 or Shift-Command-4 for screenshots. Learn more about screenshots.
17.Shift-Command-N: Create a new folder in the Finder.
18.Command-Comma (,): Open preferences for the front app.