Jeg bliver nervøs, hvis cykelkæden hopper af på vejen.

At være (jeg er nervøs) describes a state you are in (you’re nervous already).
At blive (jeg bliver nervøs) means to become / get (you move into that state). In this sentence it’s about the reaction that happens if the condition occurs.

Hvis introduces a condition (something that may or may not happen): if …
Når is typically used for something expected/regular or viewed as certain in the future: when …
So hvis cykelkæden hopper af frames it as a possibility, not a sure event.

In Danish it’s standard to put a comma between a main clause and a subordinate clause:

• Main clause: Jeg bliver nervøs
• Subordinate clause: hvis cykelkæden hopper af på vejen

So the comma marks the boundary before the hvis-clause.

Because hvis introduces a subordinate clause, and Danish subordinate clauses keep the basic Subject–Verb order:

• hvis cykelkæden hopper … (subject cykelkæden
  • verb hopper)

Inversion (verb before subject) happens in main clauses after something is fronted, not inside hvis-clauses.

cykelkæden is a compound noun:

• cykel = bicycle
• kæde = chain

kæde is common gender (en kæde), and -en is the definite ending, so:

• en cykelkæde = a bicycle chain
• cykelkæden = the bicycle chain

Danish often expresses the by adding a suffix instead of a separate word.

hopper af is a common verb + particle combination (like an English phrasal verb). It means the chain jumps/slips off.

The verb is hopper and the particle is af. In Danish, these particles often appear after the verb:

• kæden hopper af Not: kæden afhopper (that’s not how it’s formed).

Danish commonly uses the present tense for future or hypothetical situations when the context makes it clear. In conditional clauses with hvis, present tense is normal:

• hvis … hopper … = if it jumps off (at some time)

You don’t need a special future form here.

Because the adjective agrees with what it describes. Here it describes jeg (a single person), so you use the basic form:

• jeg bliver nervøs

nervøse is used for plural or definite contexts, for example:

• De bliver nervøse (They get nervous)
• den nervøse mand (the nervous man)

på vejen means on the road / on the way (location or during the trip).
i vejen usually means in the way / blocking.

So på vejen fits with something happening while you’re riding/travelling, not something obstructing.

Then Danish requires inversion in the main clause (verb before subject):

• Hvis cykelkæden hopper af på vejen, bliver jeg nervøs.

So the verb bliver moves before jeg because the sentence starts with something other than the subject.