What are Japanese counters?
In Japanese, you cannot simply say "three books" or "five birds" — you must use a specific counter word depending on the category of thing you are counting. This system (助数詞, josūshi) is one of the most distinctively Japanese features of the language, with parallels in Chinese and Korean but nothing quite like it in European languages.
Japanese has hundreds of counters, but about twenty are used daily. Learning the most common ones lets you handle the vast majority of counting situations you will actually encounter.
The most essential counters
〜本 (hon — long, cylindrical objects): Pens, bottles, trees, rivers, roads. 鉛筆三本 (three pencils). 〜枚 (mai — flat, thin objects): Paper, plates, shirts, slices. 紙一枚 (one sheet of paper). 〜匹/頭 (hiki/tou — small/large animals): 猫二匹 (two cats), 馬三頭 (three horses). 〜冊 (satsu — bound volumes): Books, notebooks. 本五冊 (five books). 〜台 (dai — machines, vehicles): Cars, computers, TVs. 〜杯 (hai — cups/glasses): コーヒー一杯 (one cup of coffee). 〜人 (nin/ri — people): 一人 (hitori), 二人 (futari), 三人 (sannin).
The counters for people are the most irregular in Japanese: one person is 一人 (hitori), two people is 二人 (futari) — both use unique readings rather than the standard number + nin pattern. From three people onwards, the pattern becomes regular: 三人 (sannin), 四人 (yonin), etc.
When you do not know the counter
Japanese has a general-purpose counter (〜つ, using native Japanese numbers: ひとつ, ふたつ, みっつ...) that works for most objects when you do not know the specific counter. It sounds slightly imprecise but is entirely grammatical and widely used by native speakers who momentarily forget the exact counter. When in doubt, use 〜つ.
Learn Japanese writing systems
Hiragana and katakana are the foundation for reading counters in context.