Covering faults of others is mubaah (permissible but not recommended).
Is recommended all the time.
The sins of criminal can also be covered up from the authorities under this principle.
Whether you cover up someone's sins or not depends on where the benefit lies.
As for the one who is morally upright, we sometimes cover up his/her sins.
The knowledge here refers to sciences but not arts or languages.
It refers to religious knowledge (knowledge of Allaah's legislation).
If a person knows Allaah's legislation, he/she is more likely to put it into practice and so go to Paradise.
He intends to do a good deed but cannot do so due to lack of ability.
He intends to do a good deed but does not do so out of laziness.
He intends to do a bad deed but does not do so because someone else prevents him.
He intends to do a bad deed but leaves it out of fear of Allaah.
He intends to do a bad deed but leaves it, not out of fear of Allaah but due to some other reason.
The pious people
People who perform miracles
Pious abstemious individuals who devote themselves to fasting
They are people of ihsaan
They are people of taqwaa (being mindful of Allaah's commands and prohibitions)
They are givers of charity
They are extreme worshippers