You can turn your mkv into something suitable for editing with ffmpeg using a command like ffmpeg -i -c:v dnxhd -profile:v dnxhr_sq -c:a pcm_s16le, which will give you pretty decent quality without insane file sizes, and uncompressed audio which will also be easier to cut. You can use a bunch of different software for this and there are a lot of good codecs, the most common ones used professionally being DNXHD and Apple ProRes. Think in terms of your video file now just being like a physical roll of film, where you can hold it up to the light and view each frame individually. Step 1: Import iMovie exported video files Open up the software on your computer and click '+ Video' to load iMovie video file that you desire to convert or directly drag and drop video file to the program. If you output it to an intraframe-only format - it's only the 'still frames' - then the files become huge but they are also much quicker and easier to work with. This is really hard for video editing software to cope with since it has to calculate the frame you're on from all the steps in between. If you have an mp4 file containing h264 video it is what's called a 'long GOP' format, with a single 'still frame' and then a bunch of frames described in terms of how things change in the scene. Okay, there are two things going on - mkv, mp4, mov etc are just the container format, that describes what the file contains and how it's put together.

Imovie to mp4 converter