Articles
Japanese has none — no the, no a, no gender, no number to agree with. 本 hon is "book", "a book", "the book" or "books", and the sentence sorts it out. What does the work instead is the particle on the noun, and a pointing system that is more precise than English articles ever were.
Definiteness lives in は vs が
| 猫がいます | There's a cat. (new information) |
| 猫はいます | The cat is here. / As for the cat, it's here. (known) |
| 男が来ました | A man came. |
| 男は来ました | The man came. |
は marks something already in play; が introduces something new. That is the same distinction English spends two articles on. See Prepositions for the full は/が story.
こそあど does "this / that"
Where English says "the book", Japanese often says which book:
| この本 kono hon | this book (near me) |
| その本 sono hon | that book (near you) |
| あの本 ano hon | that book over there (near neither of us) |
| どの本 | which book |
That three-way split is finer than English "this/that", and it maps onto the conversation, not just the room: その is also "that thing you just mentioned".
Counting is where "a" hides
To say "a book" as a quantity, you use a counter:
本を一冊買いました — I bought one book (冊 counts bound volumes)
See Nouns & gender for the counter system, which is the closest thing Japanese has to an indefinite article.
What you gain, and what you lose
The gain is enormous: no gender to memorise per noun, no plural agreement, no article to get wrong. The loss is that Japanese demands context. A bare 本 tells you nothing about how many or which; you are expected to have been listening.
Learners coming from Romance languages find this liberating. Learners going the other way — Japanese speakers learning English or French — find articles one of the hardest things in the language, because nothing in their grammar prepares them for a decision that must be made on every single noun.
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