I disagree. Nevím, jak se jezdí na koni. not only means that you cannot ride a horse but it also means that you have no idea if you sit on it facing forward, backward, hanging from under its belly or if you sit on it at all. in English the closest translation might be I DO NOT KNOW HOW A HORSE IS RIDDEN.
If I understand “neumím” correctly, “I do not know to ride a horse” is not a correct translation — this is strangely worded, but I would read it to mean something like, “I do not know when I have to ride a horse” or “I do not know when I must ride a horse...”
“To know HOW...” in English, means, in this sense, “to be able”, or “can”. That is, I have learned to ride a horse, and I have that skill. I think that’s what they are getting at in Czech with “uním” / “neumím”.
in english you definitely need the "how!" and i have never heard of umět having to do with time/when to do something. i think "to know how to" make sense.
I would say no. “To horse ride” isn’t really a verbal construction that works in English; you would have to say “to ride a horse” (and same re: to ride a motorcycle, etc.).
“Horseback riding” is a verbal construction that you do occasionally hear, but I personally don’t like “to horseback ride” either in the infinitive (although “to ride horseback” would be grammatically acceptable, I suppose, as would, e.g., “We’re going horseback riding”). In any event we would then be steering away from the simplest option, which is “to ride a horse”.