"You can't."

Two of my favorite words. Okay, not really. But, I seem to find myself using them quite a bit when dueling. Just at Regionals two weeks ago, I could list at least three examples:

You can't search with Dupe Frog if it was used in a Synchro Summon - Round 3.

You can't Divine Wrath Blackwing Armed Wing's effects - Round 6.

You can't bounce a Synchro Monster to the Extra Deck with Swap Frog - Round 8.

You can't activate Cold Wave after Pot of Duality - Round 9.

There, that's four examples. But, the moral of the story is that knowing your rulings (or simply paying attention) can give you a huge edge in a duel.

For instance, I was able to win a duel against that Burn deck because my opponent thought that they could negate Armed Wing's ATK boost effect with Divine Wrath. He had other attack-stopping cards, but it was too late to activate them, since he let the battle enter the Damage Step.

Also, I wouldn't have been able to win against my Frog Monarch opponent if he didn't misplay so badly. He Brain Controlled my Goyo Guardian and thought he could bounce it with Swap Frog. It's an interesting story, actually. I explained that he couldn't return Goyo Guardian to the Extra Deck as a cost for Swap Frog, since the Monster needs to return to his hand, but it can't, being a Synchro Monster. We called a judge, and I explained the whole cost thing again. Two more judges ended up showing up, and the three of them moved to the side of the room to discuss the ruling. When they came back, they basically told my opponent exactly what I said in the first place, and I ended up winning the duel and the match.

You don't have to know every single ruling in the game, though that would be helpful. But, you should just have general knowledge of game mechanics and rulings on commonly-used cards. If you come across a card that you're unsure about, check the rulings for it. It's really not that hard.