Surah Maryam Tafseer

Then, verily, We know best those who are most worthy of being burnt therein.

Then, at this point Allah attaches one piece of information to another.

The meaning here is that Allah best knows which of His creatures deserve to be burned in the fire of Hell and remain there forever and who deserves to have his punishment doubled.

This is as He says in the Ayah that was previously mentioned,

قَالَ لِكُلٍّ ضِعْفٌ وَلَـكِن لاَّ تَعْلَمُونَ

He will say:"For each one there is double (torment), but you know not." (7:38)