Suppose your wife, I know you don't have one yet! but just imagine you do, asks you politely to buy a loaf of bread on your way back home and you know that if you don't, she won't get mad or sad at all.

Now suppose that sh easks you to buy a loaf of bread and if you don't, she'll get mad at you.

Now suppose if you don't, she'll kill you.

Will there be any difference in saying the sentence I ... buy a loaf of bread on my way back home ?

I bet there will be.