The mass of a block is 8g and the volume is 4ml.
a) calculate the density of the block. show your work.
b) will it sink or float in water?
c) is this block more likely made of styrofoam or more likely made of metal?
d) why?
(i am getting this some, i just mostly do not quite get how i'll know if the object will either float or sink.)

If the object's density is more than the density of water, it will sink in water.
If its density is less than the density of water, it will float in water.

The density of water is 1 g/mL .

The density of anything is

                  (mass of a piece of it) / (volume of the same piece) .

For this block:        (8 grams) / (4 mL)

                       =    (8/4)  g/mL

                       =        2  g/mL

Its density is 2 g/mL. 
2 g/mL  is more than  1 g/mL 
So the block will sink in water.

Based on our life experience, we would have to guess that the block
is more likely to be metal than styrofoam, because we know that metal
sinks in water and styrofoam floats on water.