(And because of things like this, teaching grammar rules tends to be a bit of a dick. From personal experience, I think the best way to pick up how to write is to read a lot and imitate.)
I agree. I also think this applies to writing style too, especially when developing as a writer. Although I wouldn't recommend beginning with 'The Eye of Argon by John Theis' as the basis for developing a writing style.
http://www-users.cs.york.ac.uk/~susan/sf/eyeargon/eyeargon.htmI could suggest the 'write what you know' rule, however that can be limiting in a fantasy or s/f setting. How many people know how to pilot a space ship or ride a dragon? (and research can only go so far
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That rule still has merit though as there are characters, conversations and situations that can be drawn from most people's lives and utilized in their writing.
Also, write what you want to write. Even if the npc is possibly a 'insert appropriate gender superhuman/demi-dragon/nymph' ism. If the writer isn't engaged in the characters and story then the readers are unlikely to be.