How Total derivative got converted into partial derivative?

Question

While studying the book Heat and Thermodynamics by Zemansky and RH Dittman, in the topic 'equation for a hydrostatic system' (page no. 88) it was given

in equation 4.12, when we take Pressure P constant, how the dv/dt (marked in red sqaure) got converted into partial derivative? (marked in red square)

Urb · Accepted Answer

Write
$$dV=left(frac{partial V}{partial P}right)_TdP+left(frac{partial V}{partial T}right)_PdT,$$
then
$$frac{dV}{dT}=left(frac{partial V}{partial P}right)_Tfrac{dP}{dT}+left(frac{partial V}{partial T}right)_P.$$
If the pressure is constant $dP=0$, and the first term drops out
$$frac{dV}{dT}=left(frac{partial V}{partial T}right)_P.$$

How Total derivative got converted into partial derivative?

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