Japanese counter words (助数詞) explained
In English, you count things with plain numbers: one cat, two pens, three books. Japanese attaches a small word to the number that changes with what you're counting. These are counter words, called josuushi (助数詞) in Japanese.
So one cat is 一匹 (ippiki), but one pen is 一本 (ippon), and one book is 一冊 (issatsu). Same number, three different words.
The counters you actually need first
The famous tuna problem
The counter follows the shape and state of the thing, not the thing itself. A tuna swimming in the sea is 一匹 (an animal). Caught and whole at the market, it becomes 一本 (a long thing). Cut into a sashimi block it's 一柵, and the slice on your plate is 一切れ. Same fish, four counters.
Why readings change: ippon, nihon, sanbon
The hard part isn't choosing the counter, it's the sound changes. 本 reads as hon, but one is ippon, three is sanbon, six is roppon. Each counter has its own pattern of exceptions, and they only stick with practice.
How to actually learn them
Don't memorize lists. Learn one counter at a time, drill it until the exceptions feel natural, and let spaced repetition bring it back right before you'd forget. A few minutes a day covers the counters you'll meet in real life.
Counters, drills and spaced repetition. First 21 counters free, then one purchase and it's yours forever.
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