There is something I don't understand about thermodynamic potentials.
We know that the necessary condition for a system to evolve is an increase in entropy. For a system kept at constant volume and temperature for example, this means :
$$dU = \delta Q, \hspace{7mm} dS>\frac{\delta Q}{T} = \frac{dU}{T}$$ $$\Leftrightarrow TdS > dU$$ $$\hspace{2cm}\Leftrightarrow d(U-TS) = dF < 0$$
But if $V$ and $T$ are held constant, $dV=dT=0$.
So how is $\hspace{3mm}dF = -SdT - pdV \hspace{3mm}$ supposed to be anything else than zero ?