"How many boys are swimming?"
Translation:Hur många pojkar simmar?
It's because you put pojkarna instead of just pojkar. It's just like in English, you can't say 'how many the boys' in English either. Unfortunately the machine doesn't always choose the best correction.
kille and pojke mean much the same but kille is slightly more colloquial, can refer to guys who are a bit older, and can mean 'boyfriend'.