imovie supports which formats?

Good question, Apple doesn't actually publish that info, at least not anywhere I've been able to find. I'd guess .mov and MP4 files should be fine in either 1080 HD or 4K. What codec, frame size, and frame rate is the video you're trying to import?

To convert to something iMovie should support, right-click that file in the Finder, and choose "Encode Selected Video File", then choose H.264 for the appropriate frame size.

Hi,

Here's a screen shot copy from Wikipedia showing the iMovie supported formats after Mac OS Mojave:

Edit: The Mp4 supported format must be Mp4/AAC.

-- Rich