Real export and import growth

I’ve been facing a problem at work, mostly relating to ‘real’ values of ecomonic measure.

Ok, so we know that real GDP accounts for increases due to inflation. But why do we use constant prices of goods to measure real GDP growth? How do we account for increases in real price that is not due to inflation using that method?

Now the problem I’m facing at work is trying to derive the real growth in exports-imports to include in the real GDP calculation, now th nominal imports for my country for oil has dropped in half because of the cut in prices of barrels, but the quantity has remained flat. Now how do I account for that ‘growth’ in import lines. Do I use 0% in real imports for oil because the quantity hasn’t changed at the same price last year? Or do I drop the real growth by half (as nominal) becuase the price cut was not due to deflation?

I’d appreciate any help on real GDP and real trade balance.