Haha, yes you just reminded me why English is such a crazy language. I'm glad I don't have to learn it!
To answer your questions on venom, poison and toxin, I would say:
- venom almost always comes from an animal (think snake's venom), though venom in a figurative sense could mean hurtful words (e.g. I can't believe the venom that comes from our politicians these days).
- poison is a harmful / deadly substance, e.g. arsenic is a poison.
- toxin is more a component of another substance that has harmful properties, e.g. X is a toxin found in Y.
- toxicant strikes me as a very scientific word, and I would probably not use that word unless I was a qualifed chemist or chemical engineer.
But that is just my overall thoughts as a native speaker. I am sure a good English dictionary will describe the full shades of meaning of each of those words.
By the way Jakob - your written English is very good. But you shouldn't spell alcohol with a capital A. It's not a proper noun. Good luck with your English learning, you seem very advanced.