Polish and English words have different distance. Polish has ten, ten ,tamten English has this, that that
About why here are some explanations:
What you have here, is that you had a sentence that took Accusative, and then after negating, it takes Genitive. It is very common, but if a different case is negated, the case just stays. Negated Genitive is Genitive, negated Instrumental is Instrumental, etc.
I think masculine animate nouns have (mostly) an identical Genitive and Accusative form, so does masculine personal plural, but mostly it's different.
The system rejected "i do not know this dog". Normally it does not care if I leave out punctuation and don't capitalize. I would upload a screen snap shot showing that's how I entered it if I could. Also in this module it reject "don't" and wanted "do not". Normally it accepts that.