I find it easier to visualise a number as consisting of the number's value plus an infinite number of +1, -1 pairs. Then, for me, subtracting negative numbers is just a matter of removing the appropriate number of -1's from the pairs and seeing what's left.
So 5 - -1=(5+1-1)- -1=5+1=6
(-2)x4=subtract 2 lots of +4 from the +1-1 pairs=-8
(-2)x(-4)=subtract 2 lots of -4 from the +1-1 pairs=+8