There are some things I've come to believe over the years, and now and then, I offer them up. These are not absolutely true, caveats abound, so take them in that light, and in no particular order.
1. Sucker punches win fights.
This is fairly straightforward–if you surprise somebody and nail them solidly, it can shock them into a state wherein they don't get back up to speed in time. This doesn't mean you throw your Sunday punch and then step back and smile–if you are ahead, you don't wait for the other guy to catch up, you keep going until the fight is over. As much as it takes, no more.
It's not a Roy Rogers cowboy movie. You can throw the first punch, you don't have to wait, as long as the other guy's intent is clear. If he screams, "I'm gonna kill you!" and charges in to do just that? The law will allow you to defend yourself, you don't have to wait. Han shot first, and should have ... (But: see item #10, below.)
Conversely, if you are behind, best if you can shake it off and catch up and get ahead on speed and impact. Doing this is the trick, of course.
Surprise and major force wins most of the time.
2. Big bone beats little bone.
A roundhouse punch to the other guy's head will more likely result in injury to your hand than his skull. The boxer's fracture is a common injury, and since the average man's head weighs somewhere around ten pounds, (four and half kilos) think of it like this: Put a bowling ball on a table, cover it with a dish towel, then punch it with your fist. How hard do you want to do this?
The corollary to #2 is:
3. Hard to soft, soft to hard.
The fist to the solar plexus or a slap to the head will probably result in less injury to the striker. Reconsider the bowling ball analogy. If you are going to hit somebody in the head, a slap, forearm, or shoulder is apt to be better for you than a fist.
Truth? If you are long-term martial artist who bangs a lot in practice? Chances are you will get hurt more over the years in class than you will in a street dust-up, which you are theoretically trying to avoid. A street fight lasts a few seconds. Two hours of class once or twice a week offers a lot of opportunity to get whacked. Accidentally, or on purpose? Both look the same on an X-ray.
4. Size matters.
Bigger, stronger, faster, tougher make a difference, especially if skill is anywhere close to equal. Generally, the good big fighter beats the good small one. Technique can offset a lot of this, but you have to be really good to beat Godzilla. The little old lady who watches a video and then cleans out the biker bar barehanded is ... unlikely ...
5. Keep the sun at your back.
This goes to tactical advantages, and any you can get, if you have time? Take them. Cheat.
6. Don't play cards with a man named "Doc."
If somebody wants to box or wrestle, that's probably because they are comfortable there. Always better to play your game instead of theirs. And cheat.
7. People who play with knives get cut. (And they cut other people.)
If you aren't prepared for the sight of your own blood and willing to keep going? Don't engage. The corollary here?
8. Steel beats flesh.
A sharp knife in the hands of an expert is going to be nasty. If you are barehanded, anything you do against such is apt to fall into the "Oh, shit!" category of moves. He has all the tools you have plus one. A superior weapon is what you want, and the Army dictum: You're not an ape–use a tool.
And the problem is, when that guy whips out his knife? You won't know how good he is with it until the wonder becomes moot.
9. Winning is relative.
Look up "pyrrhic victory." The old Indonesian proverb is, "In a knife fight, the loser is ashes, but the winner is charcoal." The best win? You don't fight at all. Being somewhere else is good.
Nobody is bulletproof. The best fighter in the world can slip, get blindsided, or have an off day. Or can deck somebody with one punch and the downed guy can hit his head and die. Every fight has some cost, and the problem is, you don't know what it will be going in.
Monkey dancing is far more likely to get you in trouble than walking away.
10. The law will be waiting after the fight.
You have to decide in the moment if force is applicable and how much to use. You need to know that a serious fight will almost certainly have to be justified to the police, and maybe a judge and jury. If you can't do that, it will cost you, money for sure, and maybe your liberty.
There are lots of other things, and folks who have considered this have their own lists. Just food for thought ...