Sie antwortet: Ich komme auch, und zwar mit dem Fahrrad, sofern es nicht regnet.

She replies: I’m coming too, namely by bike, provided it doesn’t rain.

Breakdown of Sie antwortet: Ich komme auch, und zwar mit dem Fahrrad, sofern es nicht regnet.

• kommen: to come
• dem: the; (masculine or neuter, dative)
• ich: I
• mit: with
• nicht: not
• es: it
• sie: she
• regnen: to rain
• das Fahrrad: the bicycle
• sofern: provided that
• antworten: to reply
• auch: too
• und zwar: namely

Questions & Answers about Sie antwortet: Ich komme auch, und zwar mit dem Fahrrad, sofern es nicht regnet.

Why is there a colon after Sie antwortet?

The colon introduces direct speech (a quoted statement), similar to English She replies: ....
In German you can also use quotation marks (and often a comma), e.g. Sie antwortet: „Ich komme auch …“. The colon is a common, clean way to signal that what follows is the exact wording.

Could I replace the direct speech with an indirect sentence using dass?

Yes. You can turn it into indirect speech/reporting:

• Sie antwortet, dass sie auch kommt, und zwar mit dem Fahrrad, sofern es nicht regnet.
  Notice that ich changes to sie, and the verb position changes in the dass-clause (verb goes to the end: kommt).

Why does it say Ich komme and not something like Ich werde kommen?

German often uses the present tense for near-future plans, just like English I’m coming too.
Ich werde kommen is possible, but it’s more explicit/future-focused and can sound a bit heavier or more formal in everyday speech.

What exactly does und zwar mean here, and why is it used?

Und zwar means something like namely / specifically / to be precise. It adds a clarification to what came before:

• Ich komme auch (I’m coming too)
• und zwar mit dem Fahrrad (specifically, by bike)

It’s very common when you first state something generally and then give the exact detail.

Why is mit dem Fahrrad in the dative, and why dem?

Because mit always takes the dative case.
das Fahrrad (nominative) becomes dem Fahrrad (dative), so you get:

• mit dem Fahrrad = by bike / with the bicycle

Could I say mit Fahrrad without an article?

Usually you include an article: mit dem Fahrrad is the standard.
Dropping the article can be possible in some set phrases or special contexts, but for vehicles German typically keeps it: mit dem Auto, mit dem Zug, mit dem Fahrrad.

What does sofern mean, and how is it different from wenn or falls?

Sofern means provided that / as long as and often sounds a bit more formal or “conditional” than wenn.

• sofern es nicht regnet = provided it doesn’t rain
  Falls is close to if (in case) and can suggest more uncertainty:
• falls es regnet = if it happens to rain
  Wenn is the most general if/when and is very common in everyday speech:
• wenn es nicht regnet

Why is the verb at the end in sofern es nicht regnet?

Because sofern introduces a subordinate clause. In German subordinate clauses send the conjugated verb to the end:

• sofern es nicht regnet (… provided it not rains)

This is the same pattern as with weil, dass, wenn, obwohl, etc.

Why does it say es regnet—what is es referring to?

In weather expressions, German uses a “dummy” subject es, just like English it:

• Es regnet. = It’s raining.
  The es doesn’t refer to a specific thing; it’s just required by the grammar.

Why is nicht placed before regnet here?

In sofern es nicht regnet, the negation nicht negates the verb/action regnet (the fact of raining). In a subordinate clause the verb comes last, so nicht naturally appears right before it:

• … es nicht regnet = … it doesn’t rain

Are all those commas necessary—especially around und zwar?

Yes, they make sense here:

• The part und zwar mit dem Fahrrad is an inserted clarification, commonly set off by commas.
• The comma before sofern is required because sofern starts a subordinate clause.