Learn Turkish: Avukat, şüphelinin gizlice dışarı çıktığını kanıtladı. - The lawyer proved that the suspect secretly went out.

This word forms the core of the factive clause you are learning. It starts with the verb root çık (to go out). We add the past/present participle -dık (which becomes -tığ due to consonant mutation and vowel harmony). Then, we add the 3rd-person possessive -ı ("his/her"), a buffer -n-, and the accusative case -ı. Literally, it translates to something like "his having gone out."

Why is "suspect" written as şüphelinin (in the genitive case) instead of just şüpheli?

In Turkish factive clauses using -dık, the subject of the embedded clause (here, "the suspect") takes the genitive case (-nin). This links directly to the possessive suffix on the verb çıktığını. Think of the literal structure as: "The lawyer proved the suspect's secretly going out."

Why does çıktığını need that final accusative -ı?

The entire clause şüphelinin gizlice dışarı çıktığını (that the suspect secretly went out) acts as the direct object of the main verb, kanıtladı (proved). Because the lawyer proved a specific, defined thing, the whole phrase must be marked with the accusative case.

Why is it just dışarı and not dışarıya (with the dative "to" suffix)?