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The usual convention for tense is that when the main story is present tense, flashbacks are past tense. When the main story is past tense, on the other hand, the first paragraph or so of the flashback is past perfect. But once the time shift is firmly established, it goes back to simple past. (Other choices are possible; this is merely the most common.)
I'm going to put in my two cents about the present/past tense observation.
All scenes in your book should sound immediate. Use the word "had" and you lose immediacy unless it is: "Jim had a wedgie, now it is gone."
examples: (Flashback scene, Torg the lizard remembers his first dragon encounter)
Most of those silly humans can't see dragons. Neither can most mammals, except cats. That's another story. This is mine, Torg...lizard extraordinaire, biter of birds. I remember the first time I saw a dragon.
My egg had been drying in the sun for hours. The sun had risen past the mountains and had climbed high into the sky. Then it had been blotted out...
OK, I can't continue that because it was just silly...
Here's the better way:...I remember the first time I saw a dragon.
My egg had been drying in the sun for hours. The orb climbed high into the sky. Suddenly it was blotted out by a shadow that stretched across the entire surface of my rock. I looked up and silhouetted against the purple sky was ...
See? Unless you are referring to an event that happens even farther back in time (My egg...) then the scene should still happen in the immediate sense of past.
I'm digging out a first person account with numerous flashbacks to check... yep, it holds as a solid argument.
Here's a snippet from " Dragon" by Steven Brust:
(with comments on time)
(immediate story, note present tense "you have")Do you know what a battlefield smells like? If so, you have my sympathy; if not, you still won't because I have no intention of dwelling on it except to say that people don't smell so good on the inside.
(flashback time, no transition...the set up was above. This part is phrased in past tense.)
We stepped over the piles of dirt (I can't call it a "bulwark" with a straight face - internal comment set in the present) that we'd spent so much time and sweat creating, and moved forward at a steady pace; not too fast, not too slow. No, come to think of it, much too fast. A slow crawl would have been much too fast.
There. Illustrated. (sort of...)
Amy