Generally speaking,
>P is not true
is the same as:
>not P
Regardless of whatever i put in place of "P".
For example:

>(not P) is not true
is the same as:
>not (not P)

>(P and Q) is not true
is the same as:
>not (P and Q)

>(P or Q) is not true
is the same as:
>not (P or Q)

HOWEVER...

>(P therefore Q) is not true
is NOT the same as:
>not (P therefore Q)

Why does this happen? Where does this discrepance stems from?