Are the trainers like T-38’s actually separate platforms, or are they derived from some real combat craft? If they’re separate, why? Wouldn’t it make more sense to base them off an F-15 or whatever the pilots are actually going to use?
My guess is that trainer jets are cheaper and easier to fly, so they’re better for beginning pilots.
Correct. They’re cheaper, smaller, and more forgiving. In addition, they’re two-seaters.
Once a pilot has learned the art of flying jets (in the trainer), then he or she begins to learn how to fly the specific model of jet that he or she has been assigned to (F-15, etc.).
Advanced trainers for more-experienced students are generally variants of base fighter designs, yes. The T-38, though, was designed from scratch as a trainer, with the F-5 fighter being a later variant - same for the T-37 primary trainer and its AT-37 ground-attack variant.
Basic trainers for beginning or intermediate students don’t generally have enough performance to be worth much in combat, although that is sometimes an advantage - old WW2 T-6’s found a niche in spotting for ground-attack planes in Vietnam, mainly because they were slow and rugged.
Two-seat variants of today’s modern fighters may eventually become the trainers of tomorrow, but for now, we’ve got a bunch of the older variants sitting around, unused because our best pilots have all the high-performance aircraft they need.
If you realize that the very first plane U.S. pilots fly is almost always a single-engine propeller plane, you’ll see the progression. You begin with a very cheap, very simple, very forgiving aircraft. As the pilot earns your trust, you let him practice more aggressive and realistic maneuvers with a slightly better airframe.
By analogy, it’s the same reason that many parents teach their kids to drive on the family “beater”, and why drivers’ ed. cars are usually tiny economy cars, and why you don’t (or shouldn’t, at least, but that’s a Pit thread for another day) see 16-year-olds driving dad’s brand-new BMW on fifteen-hour road trips.
That’s almost never the case. A trainer is not just a two seat fighter with flight controls added to the back seat, they are purpose built from the get-go. It’s not a trivial matter to add extra flight controls and not cost effective for a plane that has been out of production for some time. “Hey Charlie, throw a couple of rudder pedals, a throttle quadrand and oh yeah, a joystick in the back of that F-14.” In many cases there isn’t sufficient forward visibility from the RIO/NFO seat as it isn’t high enough. There are trainer variants of the F-15 and F-16 but they came with dual flight controls and different rear seat arrangements from the factory.