Pentaxuser - many of these issues are indeed more easily apprehended by learning how to read curves themselves. I sometimes get flack about that on forums like this one whenever people don't realize that seemingly small differences in toe shape on tiny published graphs, for example, are in fact relative to logarithmic increments and have quite significant consequences in actual image reproduction. But let me try an analogy I have used before. HP5 is like an airliner very gradually rising off the runway before it attempts to dramatically attempt to climb at a more rapid rate. The gradual liftoff is like the toe of the film. It covers a lot of ground, but without much elevation distinction. Similarly, a longer film toe will bag quite a range of shadow value if adequately exposed, but not differentiate it very well. But something is likely to be there, even if not ideally usable. That's what I refer to as "forgiving".
Now let's take a film like T-Max. It was engineered to replace several prior films, so is especially amenable to development tweaking. But in terms of its typical characteristic curve, it's more like a jet fighter launching off an aircraft carrier. It lifts off and gains height very quickly. So it's like a film with a shorter toe, launching right up onto the straight line section of the curve faster. But with that short runway, it's not so forgiving. The deep shades will be better differentiated than with longer toe films, but if you're exposure is off, you're more likely to crash. Sometimes I do that deliberately - underexpose and overdevelop TMax to get bold totally black shadows. But if I try the same trick with HP5, I just get gradual blaah muddiness down there. The value separations aren't crisp. Well, there are ways to improve that in printing; but I'm referring to characteristics of the film itself.
But per film speed examples, I should really be comparing TMax 100 to FP4. Both have a long straight line section. But even though TMX100 and FP4 have similar box speeds, in terms of crisp shadow separation, FP4 needs a full more stop of exposure to push the deepest shadows up onto the straight line section of the curve, so in effect it is the slower of the two films if push comes to shove, By this I mean high contrast scenes. Under low contrast lighting, we don't need to be so critical.
Now on to Tri-X 400. It's somewhere in between in terms of its characteristic curve.. Most people addicted to it seem to rate it at 320 or less in order to boost the deep values up the curve and get better shadow separation. In those rare instances I shoot TX400, I am deliberately after an overboard look, and tend to shoot it at full 400 speed and even overdevelop it. Of course, that risks blowing the highlights out. But that's where staining developers like pyro come into play. Of course, when Kodak decided to keep Tri-X alive, yet let Super-XX and Plus-X go extinct, they needed some kind of marketing ploy attached to nostalgia, so came up with ads touting a alleged traditional combination of Tri-X and D76, both obviously their own products. But there was a bit of mythology behind that. Most of the people whining to keep Tri-X alive were actually contact printers using the sheet film 320 speed version of Tri-X instead, and developed it in pyro all along. Why? They were of the old "thick negative" persuasion. Significantly overexpose the negative to get excellent shadow gradation, then give full development, and end up with a very dense or "thick" negative actually quite hard to print except with special low-contrast contact papers like Azo. I think some of that mentality was counterproductive; but it can be hard to break old habits.
Hope I haven't been too wordy, and that this at least gives a partial view of what is involved. And I suspect that for most serious printmakers, D76 itself has been in the rear-view mirror for a long time by now. But it is a safe middle of the road formula quite suitable for beginners.