If T is constant and V increases then P has to decrease. If P decreases, the walls do not expand if the external pressure remains the same. So if T is constant, the external pressure has to decrease if there is an expansion.I don't get how is the isothermal process done ???
If you take your example, if heat flows into a gas, T increases and P increases if V is constant. However, if the external pressure is constant, then an increase in V continues until P is equal to the external pressure. So this describes a constant pressure expansion.