Trim MP4 video downloaded from YouTube in Keynote
Open the MP4 video file with the built-in QuickTime Player app. It contains fine grained controls to easily trim a video. The trim command can be found under Edit menu, or can be invoked using keyboard shortcut Command + T.



Once you have trimmed the video as per your liking, insert it into the Keynote slide. You may want to create a copy of the video if you wish to keep the original one intact.