This is an entirely useless comment to keep repeating if you're not willing to explain what a good morning is. And quite honestly, I'm not seeing what he did wrong there either, so perhaps a better explanation would be suitable here. It did not look to me like he lowered the last rep in a dangerous way. Maybe I can learn something here.
Jaack740: you're probably on the right track, simply because you've taken the step to start lifting weights. And you're performing a compound lift that can put on size and strength quickly. Those are good things.
Eating will be the thing that likely comes hardest to you, because you're so thin. Pay a lot of attention to that. Get as many calories in as you can, as long as you're not getting appreciably fatter.
As to the video itself, I second Pookie's barpath observation. It looks like you're swinging the weight out forward intentionally to miss your knees. don't do that. You need to shift your weight backwards more towards your heels. You're letting your weight shift towards your toes. That's bad, and it often causes the bar to fall forward a bit. If anything, your weight should make you fall backwards. If you ever have a rep that you miss and lose your balance forward, that's a big problem. That's the only thing I'd work on right now, driving more through the heels.