Learn Turkish: Onu tutukladıklarını bilenler bunu gizliyor. - Those who know that they arrested him are hiding it.

This is the core of your current lesson! It combines the verb tutukla (arrest) with the factive suffix -dık, the 3rd person plural possessive -ları (their), and the accusative case ending -nı. Literally, it is structured like 'their having arrested'.

Why is there an accusative ending (-nı) at the very end of tutukladıklarını?

The entire clause acts as the specific direct object of the verb bilmek (to know). Because they know this specific fact, the clause must take the accusative case. The -n- is a buffer letter that is always required when attaching a case ending after a 3rd person possessive suffix like -ları.

Bilenler means 'those who know', but where is the word for 'those' or 'people'?

It is built right into the word. The suffix -en creates a subject participle, turning bil (know) into 'who knows'. By dropping the noun it describes and just adding the plural -ler, it functions as a noun itself meaning 'those who know'.

The dictionary form is gizlemek (to hide). Why is it spelled gizliyor here?

This is due to a rule called vowel narrowing. When a verb root ends in a wide vowel like e or a (as in gizle-) and you add the present continuous suffix -yor, that final vowel narrows to i, ı, u, or ü. Therefore, gizle + yor becomes gizliyor.