How do you convert mol/dm^3 into g/dm^3 and give an example please? :)

you have dm^3 in both units so that does not change,
you need to convert mol into g
m=M*n

m=mass [g]
M=molar mass [g/mol]
n=number of moles [1]

multiply it by the number on the periodic table! (the one with lots of decimal places)