の(no) — Possession, Modification, & Nominalization

• これは私の本です。

  Kore wa watashi no hon desu.

  This is my book.

  私 (I) の (possessive) 本 (book)

• 友達の佐藤さんです。

  Tomodachi no Satou-san desu.

  This is my friend Satou.

  Apposition: 'my friend, who is Satou'

• 本を読むのが好きです。

  Hon o yomu no ga suki desu.

  I like reading books.

  読む (verb) + の turns 'read' into 'reading' (a noun phrase), then が marks it

• ・Beyond Ownership: の isn't only possessive. 日本の車 = 'a car from Japan' (Japanese car); 木の机 = 'a desk made of wood.'
• ・Replacing Nouns: 赤いのが欲しい (I want the red one) — の stands in for the noun 'one/thing.'
• ・Explanatory Endings: At the end of a casual sentence, の seeks or gives an explanation/emotional nuance: どこに行くの？ (Where are you going, then?).

Commonly confused with

The JLPT rarely asks whether you recognise a pattern — it asks which of several similar patterns fits. These are the ones の is tested against.

• は (wa) — Topic & Contrast Particle
• が (ga) — Subject Marker & Exhaustive Listing
• を (o) — Direct Object & Traversal
• に (ni) — Target, Time, and Existence Location
• で (de) — Context of Action (Location, Means, Condition)
• と (to) — And (Exhaustive), With, and Quotation
• か (ka) — Question, Or, and Uncertain Identity
• も (mo) — Also, Too, Both/And, Emphasized Quantity