1) What type of martial arts would you recommend authors learn to write hand to hand combat? 2) Which type(s) apply best to understanding large battles? 3) Are there any styles that teach sword fighting?
Whatever you want. If you want to learn a martial art, pick one that interests you. Get your base of knowledge there. The things you learn will help you understand what you’re seeing when you look at other martial arts for your characters.
I frequently say that you need to tailor your character’s martial arts training to who they are. That’s still true, and if you’re writing a character with a specific background, you’ll need to know about their martial art, but that’s not the same as being proficient in it. Learn a martial art that interests you, and stick with it. It’ll take time, and it won’t be easy, but you’ll have a broader base of experiences to draw on when the time comes to write.
If you understand your martial art, and you can remember that different martial arts solve problems in different ways, you’ll be in a much better position to look at videos of someone and understand how to write that.
None of the above. Martial arts are, “how you fight,” not how you command.
The best sources for learning about battlefield combat are history and, probably, tabletop war games. In a rare moment, I do not mean Warhammer here, I’m talking about things like Avalon Hill’s catalog of historical scenario games.
This may sound a little weird, but that kind of “simulation” war game does have value as a training tool, so (with a competent opponent) it will let you experiment with what does, and does not, work. Somewhat obviously, it does need to be tailored to the era you’re talking about. Games focused on Roman Legions will have limited applications when your setting is early modern, for example. That said, while niche, the strategy wargaming market is insatiable, so there is absolutely stuff out there that will fit your specific needs.
While wargaming will let you experiment, history will teach you what people tried, what worked, and what didn’t. It’s a vital component of this. There is a massive body of literature devoted to analyzing historical battles and the lessons learned.
If you’re asking about the experience of being on the ground in mass melee, then HEMA is probably the only martial art that will give you that experience in a safe environment.
Most Chinese martial arts, and some from other parts of south east Asian have sword forms. The overall structure is that you’ll learn the hand to hand components, and then move up to various weapon forms. It’s a holistic approach to combat training.
In Japanese martial arts, the sword is segregated off into separate martial arts: Kendo and Iaido. Just, for the record, this isn’t, “sword fighting,” it’s how you use a katana. That might sound like a minor distinction, but the katana has some serious design limitations that require special handling.
European martial arts did cover the use of swords, but most of those martial arts have been lost. What survives are training manuals. HEMA seeks to reconstruct those martial arts, but it’s not a perfect recreation. That said, HEMA will give you some historical context for sword combat, how that looked and felt.
Beyond that, fencing is an evolution from European sword martial arts, and there are a few examples like Polish Cross Cutting, which do survive to one degree or another, while mainstream fencing is an evolution from the lighter blades that came into fashion in the early modern era.
So, there are options, and this stuff is not interchangeable. If you’re looking at a pseudo-European fantasy setting, then Kendo isn’t going to work for what you want, you’ll want HEMA, Polish Cross Cutting, or to consult the surviving training manuscripts. Once you know the basics, understanding what you see elsewhere does become much easier.
So, learn what you want to start with, study actual battlefield warfare to understand how battles work, and then research the martial arts that would be appropriate for your characters. You don’t need to practice the same martial arts your characters would, but you do want a base to be able to understand their training.