@Paula741,
YouTube has converted many of their streams to DASH format, where video and audio is delivered to the computer separately and as a series of short files, instead of coming as one unified file containing the entire thing.
That works well for live streaming on the web, but not so well for downloading a video for later use.
The
ChrisPC VideoTube Downloader is able to download DASH streams from YouTube. It downloads all the video pieces and automatically grafts them together into a single file, then does the same for all the audio pieces, then uses FFMPEG to graft the audio and video together.
The free version of the software has built in limitations though (it's still pretty useful). You'll need to pay for the fully functional version.
I use the free version currently. But I'm beginning to think about paying for the full version.
If you use one of the many Java-based downloaders out there, make sure it can download YouTube's 720p MP4 streams (the largest size YouTube still offers in a non-DASH format). Also the 480p FLV streams (second largest) for those videos that are too small for 720p.