Learn Hardcore German: Ich lese nebenbei im Café, während ich auf den Bus warte. - I read on the side in the café while I wait for the bus.

Because während introduces a subordinate clause. German requires a comma to separate every finite subordinate clause from the main clause. Here, the subordinate clause is während ich auf den Bus warte. You could also flip the order: Während ich auf den Bus warte, lese ich nebenbei im Café.

Why does the verb go to the end in ... während ich auf den Bus warte?

Subordinate clauses in German use verb-final word order. So the conjugated verb warte comes at the end: während ich auf den Bus warte. In other tenses the same rule applies, e.g., während ich auf den Bus gewartet habe (perfect).

Why is it auf den Bus warten and not für den Bus or auf dem Bus?

The verb is the fixed expression auf etwas warten and it always takes the accusative: auf den Bus.
für den Bus means “for the bus” (in the sense of on behalf of or intended for) and is wrong for “to wait for.”
auf dem Bus (dative) would literally mean “on top of the bus.” If you mean on the bus as a location, German says im Bus.

What case is used in im Café, and why?

im is the contraction of in dem (dative). With in, you use:

Dative for location (no movement): im Café = “in the café.”
Accusative for motion toward: ins Café (from in das Café) = “into the café.”

Where can I place nebenbei in the sentence?

It’s flexible within the middle field:

Neutral: Ich lese nebenbei im Café, …
Also fine: Ich lese im Café nebenbei, … (slightly more emphasis on the place)
Fronted for focus: Nebenbei lese ich im Café, … (puts “incidentally” in focus) All are grammatical; position tweaks nuance and emphasis.

What exactly does nebenbei mean here, and are there synonyms?

Here nebenbei means “casually / on the side / incidentally (while doing something else).” Close synonyms:

nebenher (colloquial)
zwischendurch (“in between,” “in the meantime”) Note: nebenbei can also mean “by the way” in conversation (nebenbei bemerkt), but in this sentence it’s about doing something in parallel.

How do I express the English progressive “I am reading” in German?

German usually uses the simple present: Ich lese. To stress “right now,” add gerade: Ich lese gerade. In some regions/colloquial speech you’ll hear the “am-progressive”: Ich bin am Lesen, but it’s not standard everywhere.

Could I replace während with solange, wenn, or bis?

solange = “as long as” (emphasizes duration): Solange ich auf den Bus warte, lese ich im Café.
wenn = “whenever/if” (general or conditional): Wenn ich auf den Bus warte, lese ich im Café.
bis = “until”: you’d rephrase as Ich lese im Café, bis der Bus kommt.
For two simultaneous actions, während is the most direct choice.

Can I use währenddessen instead of the subordinate clause?

Yes, but you must use two main clauses (no verb-final clause after it):

Ich warte auf den Bus. Währenddessen lese ich im Café. Don’t say …, währenddessen ich …—that’s ungrammatical.

Why is it den Bus and not der Bus or dem Bus?

Because warten auf takes the accusative. The masculine noun Bus declines:

Nominative: der Bus (subject)
Accusative: den Bus (direct object; here after auf)
Dative: dem Bus (indirect object/location; e.g., im Bus)

What about Café vs Cafe vs Kaffee?

Café (or Cafe) = the place/establishment; both spellings are acceptable, Café with the accent is more common.
Kaffee = the drink (coffee).
All nouns are capitalized in German: das Café, der Kaffee.

What happens if I start with the während-clause?

The main clause still follows the V2 rule, so the finite verb comes before the subject:
Während ich auf den Bus warte, lese ich nebenbei im Café.
Not: ✗ Während ich auf den Bus warte, ich lese …

Is the adverb order nebenbei im Café correct? I heard about “time–manner–place.”

Yes. The neutral order is Time–Manner–Place. Here there’s no explicit time, so Manner (nebenbei) before Place (im Café) fits: Ich lese [Manner] nebenbei [Place] im Café.
With a time adverb: Heute lese ich nebenbei im Café.

Can während also be a preposition instead of a conjunction?

Yes. As a preposition it takes the genitive: während des Wartens = “during the wait.”
You could say: Während des Wartens lese ich im Café. This is a bit more formal than using the clause während ich warte.

Should I say im Café or in einem Café?

Both are fine; it’s about definiteness and context.

im Café = in a specific/known café (the one you’re in).
in einem Café = in some café (unspecified).
Similarly with the bus: auf den Bus (the bus you’re expecting) vs. auf einen Bus (any bus).