Learn Dutch: De dove hoorde de auto niet. - The deaf person did not hear the car.

In Dutch, you can turn an adjective into a noun to talk about a person by adding an -e and putting de in front of it. So the adjective doof becomes the noun phrase de dove (the deaf person).

Why did the spelling change from an f in doof to a v in dove?

This is a standard Dutch spelling rule. When a word ends in a long vowel sound followed by an -f (like doof), adding the -e ending makes the f change to a v. You also drop one o because the syllable is now open, keeping the vowel sound long.

How do we know de dove means one deaf person rather than deaf people in general?

You can tell it is singular because the verb hoorde is singular. If the sentence were about plural deaf people, the noun would get an -n ending (de doven) and the verb would be plural (hoorden).

Why is niet placed at the very end of the sentence instead of right after the verb?

When you negate a sentence that has a specific direct object (like de auto, a specific car), niet must come after that object. Since there is no other extra information in this short sentence, niet simply lands at the very end.