It depends, but actually you want usually explaining your rules when something is happening, not in advance and ideally you shouldn't need to explain anything and people should be able to figure out rules on their own just from visuals and short comments. Nobody really wants to read a story that opens with lengthy explanations of the rules, unless it's done really well and/or interesting and not in wall of text info dump. You can set some rules before, but again, the most important thing is feeding those information to your audience naturally and in small doses.
"ideally you shouldn't need to explain anything and people should be able to figure out rules on their own"
On their own? yea of course, go, tell a story, say very little and expect ppl to figure it out on their own! yes!
I don't "want" 5 paragraphs of a whole monologue as you imply in "Nobody really wants to read a story that opens with lengthy explanations of the rules", something really short and explanatory would do the trick, you don't need to explain the whole thing for 10 minutes and as i said, again, you need a base first so everything that comes after becomes something cohesive, so what you said "You can set some rules before, but again, the most important thing is feeding those information to your audience naturally and in small doses." is just exactly what i said with other words so thank you, whoever i don't think that feeding information little by little is the MOST IMPORTANT, is just as important as the base rules, one wouldn't work without the other.
And about the cantaur no nayami thing, i was talking about the anime so srry about not explaining that. On the anime, as i remember, there's a short story at the beggining explaining everything really fast, but in a way everyone can understand.
You said "We got explanation in first chapter", but i was talking more about the characters, "Why the girl just believed so fast?" that's it, since i actually don't remember seeing anywhere that the girl was also a monster and that's why she didn't get scared(and also, when i posted that comment i've only read the first chapter and in the first chapter, the girl being a monster is unsaid).
"Did you ever heard of love at first sight, especially when girl is very cute..." saudhsaoidsahiudsah yea sure "love at first sight", i don't believe that. I believe that love is a strong, unbreakable feeling and you can't just "I looked at her and i knew... i love her" no, this is passion, there's difference. Love is something that comes with time, absolute, it doesn't burns like passion, it's a gentle and scary feeling. A passion's burning makes you blind. So i'd say that part is an opinion thing.