Relative clauses (ilgi tümcecikleri) are clauses that modify a noun, providing more information about it, similar to "who," "which," "that," or "whose" in English. Turkish primarily uses participles to form relative clauses.

The participle acts as an adjective describing the head noun of the main clause.

1. Using the -en/-an Participle:

This participle is used when the noun being modified is the subject of the action in the relative clause.

• Kitap okuyan çocuk (The child who is reading a book / The book-reading child)
• Hızlı koşan at (The horse that runs fast)
• Dün gelen misafir (The guest who came yesterday)

2. Using the -dik/-dık (and its variants) Participle:

This participle is very versatile. It's typically used when the noun being modified is the object of the action in the relative clause, or when possession is involved. It often requires possessive suffixes to indicate the subject of the relative clause verb.

• Benim okuduğum kitap (The book that I read/am reading)
  (Here, kitap is the object of okumak; benim indicates 'I' did the reading)
• Senin gördüğün film (The movie that you saw)
• Onun yazdığı mektup (The letter that he/she wrote)
• Kapısı açık olan ev (The house whose door is open) - Here -olan is used with a possessive structure.

The noun modified by a -dik participle can also take case endings: Sevdiğim kıza bir hediye aldım. (I bought a gift for the girl I love.)

3. Using the -ecek/-acak Future Participle:

Used when the action in the relative clause is in the future.

• Yarın gelecek adam (The man who will come tomorrow)
• Okuyacağım kitap (The book that I will read) (okuyacak + -ım)

Relative clauses in Turkish precede the noun they modify, forming a single noun phrase.