Learn Hardcore Turkish: Saklambaç oynayan çocukların neşesi her yeri sarıyordu. - The joy of children playing hide-and-seek was spreading everywhere.

The suffix -YAN (or -YEN after front vowels) is the present participle suffix. It attaches to the verb stem oyna- (to play) to form oynayan, meaning playing or who is playing. So saklambaç oynayan çocuklar literally means hide-and-seek playing children, i.e. children playing hide-and-seek.

Why does the phrase saklambaç oynayan come before çocukların in the sentence?

Turkish is a head-final language, so modifiers—adjectives, participles or relative clauses—always precede the noun they modify. Here saklambaç oynayan modifies çocukların, so the participial phrase appears before the noun.

What is the function of -ın in çocukların?

The suffix -IN marks the genitive case (possession). In çocukların neşesi, çocukların means of the children (children’s). The possessor noun takes -IN to show that neşe (joy) belongs to çocuklar (children).

Why does neşe take the suffix -si (neşesi)?

When a noun is possessed by a genitive-marked possessor, the possessed noun takes a possessive suffix. Here, neşe (joy) takes the third-person singular possessive suffix -SI, becoming neşesi, meaning their/children’s joy.

What tense and aspect does sar + -ıyordu (sarıyordu) express, and what does it mean?

The suffix -YOR is the progressive aspect marker (ongoing action) and -DU is the past tense marker. Combined as -YORDU (with a vowel ı for harmony), they form the past continuous tense. Sarıyordu thus means was spreading or was covering.

Why is her yer in the sentence her yeri sarıyordu and why does it take -i (her yeri)?

Her yer means everywhere. Sarmak (to cover/spread) is a transitive verb that takes a direct object. When that object is definite or quantified (as her yer is), Turkish marks it with the accusative suffix -i. So her yer becomes her yeri.

Why isn’t saklambaç marked with an accusative suffix in saklambaç oynayan?

In Turkish, only definite direct objects take the accusative suffix -(y)I. Indefinite objects remain unmarked. Here, saklambaç (hide-and-seek) is used in a general, indefinite sense (playing hide-and-seek), so it stays without an accusative suffix.