I ask this as a a spades player for years. I understand how every aspect of the game works bar this one. How is tat I can earn 4 extra tricks having bet 3, and get 34 points, but that when I win 3 extra tricks having bet 4, I get my 43 points but get dropped by 57 due to sandbags? Can someone explain how I know how many tricks I'm allowed to win as surplus on top of what I already have? Thanks.

Hello,
it is quite simple, you can get 9 sandbags, and as soon as you earn your 10th, you lose 100 points.

If your score is positive, you can keep a track of them quite easily by looking at the last digit. For example, your score is 43, means that you have 3 sandbags, and you can take 6 more. As soon as you go above 9, you will lose 100 points.

But surely, ias in my example above, if I have earned 43 points, I should be able to earn another 6 tricks? I got sandbagged for that

What I believe is confusing you is that this rule applies for the entire game, not just for one round.

In your example, it doesn't matter whether you win 43 or 49, because we don't know how many sandbags you won before that.
Let me take another example so you can understand it better. Let's say you have 85 points in total, and the next round is about to start. You bet 4 tricks, and win 7 in total, so 3 extra. This is completely fine, you'll win 4 tricks +3 sandbags, so your score is now 128.
However, in this next round where you have 128 points, count it as 120 points and 8 sandbags, you are no longer allowed to do the same thing. If you bet 4 and win 7, you'll win 4 tricks, and your second sandbag will cost you 100 points, with an additional sandbag as well since you won 3 extra.

I hope that makes it more clear. You can take a game with 3 players as practice, since in a team bets of both players are counted together, so that's also what perhaps confused you.

Thanks so much for the clarification. That makes perfect sense. I thought the rule applied per round!