1 mL equals 1000 milligrams

Answer 1

Milligram is a unit of mass, milliliter is a unit of volume, so these really measure different things. For a specific substance, you can do the conversion if you know the substance's density (mass per unit of volume).

Answer 2

ml is a unit of volume while mg is a unit of mass. They cannot be compared.
It depends on the density of the liquid you are measuring.

Answer 3

It can be a very rough approximation, but in the right order of magnitude. If you know the density, density is mass/volume

Answer 4

This cannot be sensibly answered. Milliliters (mL or ml) and liters (L) are measures of volume. Grams (g), kilograms (kg) and milligrams (mg) are measures of weight or mass.