https://youtu.be/tt-Y8rPOPHE?t=70 I'm trying to follow this video having never used Blender to model something before, and at 1:12, he selects 3 vertices and drags them to the upper left. The rest of the object stays the same. When I try to do it, every other vertex moves, to a lesser extent. After some googling, I can tell you it isn't 'proportional editing'. I can find that option and see that it's not on and if I do turn it on, the other vertices move noticeably more. This is what my scene looks like before and after the transformation.
This is a cube with a Subdivision Surface modifier. I have Blender v2.82.7
I hope the info I've given is helpful, please let me know if you want pictures or info from another tab. Be explicit about how to find it, because I don't know what anything is.