If 25 micro litres, 40 mM of a substance, what would it be in nmol?

Much more information is required.

Microliters is a unit of volume. I'm not sure what mM is. Is that millimoles? Nmol is a unit of moles (nanomoles). To convert between moles and mass, the formula weight is required. To convert from mass to volume, you need the density.

If 40 "mM" is millimoles, then 40 millimoles is 40,000 micromoles and 40,000,000 nanomoles.