Mention rules, especially to a bunch of writers, and you get one of two reactions.
Some will wag their heads sagely, and say, "Yes. That's right. There are rules about writing, and you must follow them."
Others will brandish their fists and shout: "Rules? We don't need no stinking rules."
Now, here comes the shocker. Both groups are right. Writers should follow rules. Writers should ignore rules. The trick is to know when to do each.
How To Write A Flashback Scene
(This article previously appeared in Writing Hacks, my subscription newsletter for writers. Subscribe today if you want to read articles like this as soon as they are published.)
Rules say to use Past Perfect and relate the scene.
The storm had hit on a Sunday. I had been crying because Jack and I had been fighting. The wind had shattered the windows, spraying glass everywhere, but Jack had flung himself over me. That's when I had known that he loved me.
Let's break that rule. Rather than depart from the story, create a transition into the world of the flashback scene by starting out in past perfect then going to the present time of your story, tell what happened as if it were present time, then end the reminiscence in past perfect and go back to the normal tense you'd been using. Same example. Oh, and 3 is the magic number here.
The storm had hit on a Sunday. I had been miserable for a week because Jack. Jack and I had been fighting. (3 past perfect verb sentences)
The wind shattered the front windows, spraying glass everywhere. Jack flung himself over me. (Simple past tense, which is how most stories are told, for however long the scene lasts. In flashbacks, when you switch to simple past, passages are usually much longer to make them justify this flip-flop of verb tenses, but that's just another rule.)
That had shaken me. That's when I had known that he loved me, and, in that same instant, I had known that I loved him. (Back to the past perfect to end the flashback with 3 past perfect verb sentences.)
I encourage you to learn the rules. I prefer to call them tools in your writer's tool kit. Then learn how to break those same rules, when to break them, and why you're breaking them. Rule breaking just adds more tools to your writer's tool kit.