Any tips for a slow beginner?

wdym

white would have lost a knight and lost

I mean if you have to move the piece twice you do it, it will have some cons (losing a tempos) but in this line white is still better since well black's opening was unsound

what about this that is a possible opening

you move the queen mate because you have to

Well who said that you should follow the rules blindly? You should follow them within reason.    In this case the opponent moved g pawn twice which is not really developing his pieces, so moving a piece twice is certainly not the end of the world, and you even get a free pawn. So moving pieces only once applies if you opponent plays reasonable and doesn't threaten something right away like in this case.

then why do people say dont move ur pieces twice in the opening

Because you shouldn't if your opponent develops fast, in order not to be beaten in the first 10 moves. If he doesn't develop fast and does something like this, you are allowed to punish him if you can.  Moving your pieces once certainly doesn't mean leave your knight hanging because you mustn't move it again.  Certain amount of logic thinking has to be included. There are rules that are applied 90% of the time, but sometimes situation demands of us to be flexible, and make exceptions.

For instance, look at white's bishop and knight play in this game:

My conversion could be a lot better, but this is a prime example what can happen if someone makes multiple moves with one piece and forgets about developing.

I never said my advice was the only valid one. The thing is, aren't tactics also calculation exercises? I really don't like to attack people usually so let's continue this nicely.

I suggest you do tactic puzzles and lessons. Once you master that, I recommend drills because that can really improve your endgame strategies. You should also make an account on lichess.org, and that will teach beginners how the game works and some good strategies. You can also watch titled players' streams, and you can evaluate how they play.

And as for opening principles. I made one move - knight move 6. ...Na6. It is always said that you shouldn't make a knight move on the edge of the board like that Na3/Na6 or Nh3/Nh6. But that was the only move to keep me in the game. And because he moved his knight for the second time to put me in that position in the first place, it is ok. 

But for someone rated 800 for instance, they don't know when it is ok to bend the rules, and by abiding to the rules, they will be ok most of the time. Of course even a weaker player should understand that if your knight is attacked, it is probably a good idea not to let it be captured.

The rules are supposed to be guidelines to making better moves. These rules of thumb are rough outlines for a player to start with to get the basics down, and learning the nuances comes with time and practice. Also, some rules are more important than others in certain situations. The value of not randomly losing pieces, or of noting a knight is 3 points to a pawn's one, will have times where they trump the concept of moving more than once. 

My message is somewhere on this topic, but as it is 14 pages long, I will just post it again. I've written a guide on how to improve your game. Here it is:

https://www.chess.com/blog/nklristic/the-beginners-tale-first-steps-to-chess-improvement

I hope you will find it helpful.