Question

Im struggling hard to understand Subtracting Negative Numbers.

Answer 1

No, that "bigger number is negative" bit isn't right.

Once you have done what  you have to do inside the brackets (in this case, nothing), you get4--8=12.
Now we all know that two negatives make a positve, so it's 4+8=12, which makes perfect sense.

The other twop examples you gave work exactly the same way: two negatives cancel each other out and make a positive.