This is a great free software online that you can use to download youtube videos. Here are some examples:

Example:

youtube-dl -h

This will show you a list of all the options for running the commands

youtube-dl -U

Update to the latest version of the youtube-dl software.

youtube-dl -F https://www.youtube.com/watch?v=JvEP8PapMzo

This will show all the potential quality available for download for the video.

youtube-dl -f 160+140 https://www.youtube.com/watch?v=JvEP8PapMzo

This will copy the video with low quality. So 160 is 144p video and 140 is m4a at 129K for the audio

youtube-dl -f 160+140 PLqnnqjN9VI8DxJ7h0LEw28lxttHth6QsN

This will download the entire playlist low quality. So 160 is 144p video and 140 is m4a at 129K for the audio.

youtube-dl -f 136+140 --playlist-start 21 PLy_BU-vwATIx752jAkZYztMNs7bHpbsqE

This will download a playlist from position number 21 in the playlist. So starting from video 21 in the playlist the download will start.

youtube-dl --write-sub https://www.youtube.com/watch?v=gKG9p81jY4g

This will download video from with subtitles file if there is one.

youtube-dl -f 136+140 --write-sub PLzufeTFnhupygZG_sczSJ_j7efFT1Kst_

This will download the videos 720P from the playlist with subtitle file.

youtube-dl PL9zrHzoEwKpunssKCflLbAu8CunKRTkkP --write-sub --skip-download

This will download all the subtitle files without downloading the videos.