Count things and people with the right counter

You cannot say “three umbrellas” with the number alone: the number takes a counter chosen by the thing. ほん for long thin things (umbrellas, pens, bottles), まい for flat things (paper, stamps, shirts), にん for people, and つ for anything you have no counter for yet — it works up to ten and is the one to fall back on. Where the number and the counter meet, the sound often changes: いっぽん, さんぼん, じゅっぽん. The table below is the whole set; the forms that change are the ones inked in red.

Why do counters exist? Japanese nouns have no plural — ほん is both “book” and “books” — so the number needs something that says what it is counting. English does the same in a small way: “three sheets of paper”, “two cups of coffee”, “five pairs of shoes”. Japanese does it with everything, always, and the word is chosen by shape: flat, long, small, living. So it is not a rule to learn but a habit to get — the five or six counters you meet first cover almost everything you want to say, and つ covers the rest.