Hold down the Control key, and then click where you want to split the clip. Choose Split Clip from the shortcut menu that appears. Select a region of the clip beginning at one end up to the point where you want to split the clip, and then choose Clip > Split Clip.
What is the shortcut to split in iMovie Mac?
https://www.youtube.com/watch?v=YZM_DCd23o0
What are the keyboard shortcuts for iMovie?
Action Shortcut
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Play the selected item full screen Shift-Command-F
Exit full-screen view Escape (Esc) key
Loop playback Command-L
Show or hide clip information when skimming in the browser Control-Y