This is the 13047th most frequent Turkish word.
kapattığını
"Kapattığını" means "that you/they closed" or "that you/they shut" depending on the context.
The word 'kapattığını' in this sentence is the gerundium form referring to the act of turning something off, indicating reported speech.
O, dün gece ışıkları kapattığını söyledi.
He said that he had turned off the lights last night.
In this sentence, 'kapattığını' is used to denote the specific act referring to the action performed by the addressed individual in a subordinate clause.
Senin kapattığını düşündüğüm dosya hâlâ açık.
The file that I thought you closed is still open.
Here, 'kapattığını' implies a personal action which is observed or noted, describing a past completed action embedded within the main clause.