(أَلَمْ يَعْلَمُوا…) (Did they not know): In the Arabic Language, a declarative sentence can be either a statement free from negation, a statement with negation, or a statement with interrogation. The strongest form of declaration is a statement with negation and interrogation together, as the declaration, in the first type, comes from the speaker. When Allah (swt), says:

(أَلَمْ يَعْلَمُوا…) (Did they not know?): This is a form of declaration that combines both interrogation and negation, making it one of the strongest forms of declaration.

…أَنَّ اللَّهَ يَعْلَمُ سِرَّهُمْ وَنَجْوَاهُمْ..  (that Allah knows their secrets and their private conversations?): The (secret) is what a person conceals within himself and does not disclose to anyone. When it is something shared with others it would be called (private conversation). This is another act of the hypocrites against the believers. In this context, Allah (swt) emphasizes that He knows their secrets and private conversation.

…وَأَنَّ اللَّهَ عَلَّامُ الْغُيُوبِ  (and that Allah is the Knower of the unseen.): As mentioned before, The unseen is three types, one is hidden by time, the second is concealed by space. And the third is hidden by future. Allah (swt) is the Knower of the unseen. He knows the secrets, private talks, and what the hearts conceal. As He (swt) says:

  يَعْلَمُ خَائِنَةَ الْأَعْيُنِ وَمَا تُخْفِي الصُّدُورُ 

(He knows the deceit of the eyes and what the breasts conceal) (Ghafir: 19)