how to use the if function on Excel?

My problem is that when you use the rounddown function it doesn't really round down it rounds to the number closest to zero so if you're working with negatives and you want it to round to the number with the least value you have to tell the computer to round up so what I want to know is how I can use the if function to tell the computer to round down if the answer is less than zero or round up if the answer is greater than zero.

If it help this is what I'm using now =ROUNDDOWN((A1-10)*0.5,0) or =ROUNDUP((A1-10)*0.5,0)

so if it would work better to tell the computer to round down if A1 is less than 10 or round up if A1 is greater than 10 that would work too.