The notation "p --> q" means "if p, then q". For example
p = it rains q = the grass gets wet
So instead of writing out "if it rains, then the grass gets wet" we can write "p --> q" or "if p, then q". The former notation is preferred in a math class like this.
So when is the overall statement p --> q false? Well only if p is true leads to q being false. Why is that? It's because p must lead to q being true. The statement strongly implies this. If it rained and the grass didn't get wet, then the original "if...then" statement would be a lie, which is how I think of a logical false statement.
If it didn't rain (p = false), then the original "if...then" statement is irrelevant. It only applies if p were true. If p is false, then the conditional statement is known to be vacuously true. So this why cases iii and iv are true.
