Translated examples
私のかばんです。
It is my bag.
日本語の本です。
It is a Japanese-language book.
日本の車です。
It is a Japanese car.
兄の本です。
It is my older brother's book.
Meaning
の connects one noun to another noun. It can show possession, belonging, origin, category, or a simple description.
Formation
Put の between two nouns. The first noun describes or belongs to the second noun, as in 私のかばん or 日本語の本.
Usage
Use の to say whose something is, what kind of thing it is, or where it is from. It is one of the most common ways to build longer noun phrases at N5 level.
Nuance
の does not work like a sentence particle such as は, が, or を. It stays inside a noun phrase and links nouns together. The whole noun phrase can then be used with another particle.
Comparison
私は学生です uses は to mark the topic of the sentence. 私の本 uses の to connect 私 and 本 inside one noun phrase. These particles have very different jobs.
Common Mistakes
A common mistake is forgetting that の connects nouns only. Another mistake is adding の where a sentence particle is needed, such as 私の学生です instead of 私は学生です.
Related Grammar
Wa vs Ga: Topic, Subject, Contrast, and Practice
Use context to choose between は and が. は establishes what you are talking about or sets up a contrast. が can identify the person or thing that supplies the answer. Translating both as the English subject hides this difference.
も
も (mo) means also or too. It replaces particles such as は, が, or を when adding another person or thing to the same statement.