Learn Turkish: Terk edilmiş gecekonduların dayanıksız bir yapı olduğunu herkes biliyor. - Everyone knows that the abandoned shanties are a fragile structure.

In Turkish, to say 'that [Subject] is [Noun/Adjective]' as the object of a verb like biliyor (knows), you cannot attach the object suffix directly to the noun. You must use olmak (to be) as a helper verb to carry the clause suffix. So, 'that they are a structure' becomes yapı olduğunu (literally: 'their being a structure'). It uses the -duk participle, the possessive suffix -u matching the genitive subject gecekonduların, and the accusative case -nu because the whole clause is the direct object of biliyor.

Why is gecekonduların in the genitive case here?

In Turkish complement clauses that act like nouns (using -dık or -ecek), the subject of that inner clause must take the genitive case. Since the abandoned shanties are the subject of the inner clause 'the shanties are a fragile structure', gecekondu takes the plural -lar and the genitive -ın, linking grammatically to the possessive suffix on olduğunu.

Since gecekonduların is plural, why isn't the verbal noun pluralized as olduklarını?

In Turkish grammar, when a plural subject is non-human (like buildings, animals, or concepts), the verb or verbal noun typically remains in the singular form. Therefore, we use the singular possessive olduğunu rather than the plural olduklarını, even though 'shanties' is plural.