Conjunctive and disjunctive normal forms. Wikipedia says that $A \lor B$ is in CNF as it can be written as $\mathbf{True} \land (A \lor B)$.
Now it also says that $\lnot (A \lor B)$ is not in CNF. 
According to me we can write it as $\mathbf{True} \land \lnot(A \lor B)$ and it should also be in CNF. 
Even if we expand the brackets within the negation it will be in CNF 
So why is it not in CNF ?
 A: An expression is in CNF if it is a conjunction of $1$ or more conjuncts, where each conjunct is a disjunction of $1$ or more disjuncts, where each disjunct is a literal, and where a literal is either an atomic statement or the negation thereof.
$A \lor B$ fits this definition: it can be seen as the conjunction of exactly $1$ conjunct $A \lor B$, which is a disjunction of $2$ disjuncts $A$ and $B$, and $A$ and $B$ are both literals, since they are both atomic.
Note that something like $A$ fits the definition as well: it would be a conjunction of $1$ conjunct $A$ that is a disjunction of $1$ disjunct $A$ that is a literal since it is atomic.
However, there is no way to make $\neg (A \lor B)$ fit this definition, so it is not in CNF. You can immediately tell, because the only way a negation appears in a statement that is in CNF is if it is a negation of an atomic statement.
A: You have to move the negation inward--negation should only apply to individual variables.  This means you should use DeMorgan's law to do some rewriting.
See:
https://en.wikipedia.org/wiki/Conjunctive_normal_form
A: CNF must be AND's of operands that can only be OR's of (1 or more) literals. The example you show is not CNF because is an AND, one of whose operand is a NOT of something not an OR of literals.
