These abbreviations will definitely benefit you when watching YouTube videos, and you should try them.

Spacebar with letter K
These two keys are important for controlling the playback of any video you are watching, making it easier for you without using the mouse or if the mouse is disabled.
Arrow Keys
These keys are useful for fast forwarding or rewinding the video by 5 seconds. The right arrow key fast forwards while the left arrow key rewinds. The up arrow key increases the volume and the down arrow key decreases it.
Letter F with Esc key and F11 key
The F key enters full screen mode and exits it when pressed again. You can also exit full screen mode by pressing the Esc key or the F11 key.

Numbers from 9 – 0
You can use the numbers on the keyboard to navigate quickly within a video. Pressing the number 0 will take you to the beginning of the video, while pressing numbers from 9 to 1 will take you through 10% to 90% of the video.
Letter J with Letter L
This shortcut is useful when you want to jump 10 seconds forward or backward in the video. The L key moves you forward 10 seconds and the J key takes you back 10 seconds. This duration is double that of the arrow keys.
Letter M
This key can be used to mute the video quickly or restart the sound if it was muted before.
Home key with End key
These keys are used if you want to start or end the video. The Home key takes you to the beginning of the video, unlike the End key which is used to end any video and thus start a new video if you are following a playlist.

Letter C with + , – keys
This shortcut will help you if you are watching a video that supports subtitles. Pressing this shortcut will enable subtitles and pressing it again will disable them. The + and – keys are used to zoom in and out on the subtitle text.
Shift + N with Shift + P
If you are watching a playlist on YouTube, you can use the Shift + N shortcut to play the next video in the playlist, while the Shift + P shortcut will go back to the previous video in the playlist.