Hi, Maryann108,
Insert the DVD into an internal or external optical drive. Click on the DVD icon that appears on your desktop to open the DVD to access its contents architecture. You will see a folder labeled VIDEO-TS. (There will also be an AUDIO-TS folder, but ignore that one because it is irrelevant.) Open the VIDEO-TS folder. You will see several files in there, some with VOB extensions. Copy (not move) to your desktop the files with the VOB extensions. Ignore the others. The VOB files comprise the movie on the DVD in Mpeg-2 format. You need to convert these to MP4 format. You can do that with the free download, Handbrake.
You can get Handbrake here:
https://handbrake.fr/
A simple way to convert with Handbrake is to open Handbrake and do a File/Open Source. From the resulting screen navigate to your VOB file on your desktop and choose it as the source. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted Mp4 clips into iMovie. Arrange and edit them as you wish and then share out the project as a movie file on your desktop. You might be able to do the above by directly opening the DVD in Handbrake to access and convert the VOB files. My solution is the old fashioned way. :)
Once you have the vids converted to Mp4, import them all in to iMovie and work with them there. There you can do a Modify/Split at the in points and out points and thereby create a segment. You can do whatever corrections that you want and either leave the segment in the project or copy and paste the segment into a newly created project.
-- Rich