Use of auf - Lesson 10.8 Adjective endings

In lesson 10.8, there are two examples in a row that use auf. One takes the dative case and one takes the accusative case. 

Der junge Mann fährt auf dem neuen Fahrrad. (Dative)

Die jungen Eltern passen auf das kleine Kind auf. (Accusative)

Consider adding an explanation for this in the module. Adjective endings require enough attention, energy, and focus without the distraction of trying to figure out why different cases are used after the same preposition. With that in mind, consider rewriting the preceding paragraph. That is also not as clear as it could be. Thanks. 

Two related questions:
Why is it "auf dem neuen Fahrrad" and not "mit dem neuen Fahrrad"?
What is the function of the second "auf" at the end of the sentence: Die jungen Eltern passes auf das kleine Kind auf?
 

The second auf is there because aufpassen is a separable verb and the the auf goes to the end of the sentence.
You need 2 aufs -  auf jmdn/etwas [acc] aufpassen.  To take care of someone/something.

Auf is one of those prepositions that can take either accusative or dative. I don't have level 1 so I don't know if this has already been covered before this lesson, but I do think more use of the "literal" button could be made to explain things. ( although maybe the button should be called something else like "help")

Hallo RobS49 und sfpugh,

I can totally understand - German prepositions can be tricky. 

In regards to your first question, RobS49, the literal translation of the sentence would be "The young man rides on the new bike." The reason they use auf here is because you would normally sit on a bike - "auf einem Fahrrad". 
Nevertheless,  it would also be correct to say "Der junge Mann fährt mit dem neuen Fahrrad." - "The young man rides the new bike."

Sfpugh has also given a very good answer to your second question. Aufpassen is a separable verb which means it contains a separable prefix (auf in this case).  When you use separable verbs in a sentence, the main verb goes in its normal place - the second position - but the prefix goes last. However, you still need the preposition auf in the sentence as well. 
If you would like to learn more about seperable and inseperable verbs you can have a look at Module 12 lesson 12.5 and 12.6 Inseparable Verbs (Part 1 and 2).

The preposition auf (as well as über, unter, vor, zwischen, an, neben, hinter and in) can either be followed by the accusative or the dative. You can have a look at lesson 4.9 Prepositions with the Accusative for more information. 

I hope this helps! I will still pass your feedback on to our development team to look at. 

Viele Grüße
Julia

Thank you, both! One more thought for the development team on this module. On the recording for Adjectives with indefinite articles (ein, eine), the nominative says "ein roter Wein". However, the speaker clear says "ein 'rot' Wein". When it's clear he is doing his best to enunciate, it seems odd to have a recording that isn't consistent with the spelling.

Well spotted, I never noticed that. I did notice that the combined noun "Rotwein" appears 8 times in the course, and perhaps the speaker defaulted to that, but it should be spoken as written.
Maybe "Ein roter Wein" was a bad example to choose because of "Rotwein"?

That's a great point. I hadn't thought about "rotwein" in that way.

Hi RobS49,

thank you very much for pointing that out - I will also pass this on. Our apologies for the mixup!

Viele Grüße
Julia