"naar" typically indicates direction, meaning "to"
"na" always means "after"
It's as if two of the meanings of "nach" in German were split into two words.
However, I'd say "naar" is more widely used as an indicator of direction than "nach" in German.
Where you'd say "Ich gehe in den Park." in German, you'd say "Ik ga naar het park." in Dutch.
"naar" also reflects "nach" in prepositional phrases like "Dat ruikt naar..." (Das riecht nach...) or "Ik zoek naar..." (Ich suche nach...).
Delighted with this answer, I have discovered, alas, that it is simplified. For instance, rkd.nl uses 'naar' in the sense of a copy of (a painting by), where there is less a sense of direction. You can find a fuller explanation of the differences here: https://onzetaal.nl/taaladvies/advies/na-naar
Unless one uses a translation app, the web page your link points to is all in Dutch. That's fine if one already is fluent speaking Dutch and merely wants to improve one's grammar. But for learners of the language these explanations require more language knowledge than is currently accessible to most students at this juncture.
The reason the microphone questions don't work with numbers is because Duolingo uses Google's speech engine to analyze the user's speech. Google typically transcribes numbers in numerical form, so when Duolingo compares the generated speech with the correct answer, the numbers don't line up. Fixing this problem would take just a few lines of code, and yet....
In speaking exercises, Duo recognises NO SPOKEN NUMBERS. EVER. This is a major glitch in the system, which ruins my scores. It is not a matter of my pronunciation, nor microphone. It is consistent with numbers, and also with the second time a word, like "de" appears in a sentence.