There are two very useful keyboard shortcuts for watching YouTube videos which almost never mentioned. They are the comma and the full stop.

Both work when you have paused a video (which you can do with the pause button or by pressing Space or K).

The first (,) nudges you back a frame in the video. The second (.) nudges you forward one frame. That makes them great allies if you want the video to stop at a very particular place to look in detail at what is on screen or to get just the right screenshot.

J and L, by the way, move you 10 seconds at a go.

And if you then want to link through to a specific point in the YouTube video which you have located? See How to link to a specific time in a YouTube video.