Hvis det regner, tager jeg min regnjakke på og cykler med hjelm på cykelstien.

Hvis means if, so it introduces a condition: this happens if it rains.

• Hvis det regner = If it rains
• Når det regner = When it rains

A native English speaker should think of it this way:

• hvis = conditional, not guaranteed
• når = expected/repeated situation, more like when

So this sentence presents a condition, which is why hvis is the natural choice.

In Danish, weather expressions often use det as a dummy subject, just like English uses it.

So:

• det regner = it is raining
• det sner = it is snowing
• det blæser = it is windy

The det does not refer to a specific thing. It is just there because Danish clauses usually need a subject.

This is a very important Danish word-order rule.

When a non-subject element comes first in a main clause, the finite verb must come before the subject. This is the Danish V2 rule.

Here, the whole clause Hvis det regner comes first. After that, the main clause begins, and the finite verb takes second position:

• Hvis det regner, tager jeg min regnjakke på ...

Not:

• Hvis det regner, jeg tager ...

You can compare:

• Jeg tager min regnjakke på, hvis det regner.
• Hvis det regner, tager jeg min regnjakke på.

Both are correct, but when the if-clause comes first, inversion happens in the main clause.

Because tage på is a verb + particle combination, often functioning like put on in English.

So:

• at tage en jakke på = to put on a jacket

In a main clause, the finite verb appears early, and the particle often comes later:

• Jeg tager min regnjakke på.

This is similar to English:

• I put my rain jacket on.

The på belongs with tager, but Danish often splits them in normal sentence structure.

Because Danish normally does not use an article together with a possessive like min, din, hans, etc.

So:

• min regnjakke = my rain jacket
• not en min regnjakke
• not den min regnjakke

This works like English:

• my jacket, not the my jacket

Because the subject is the same in both coordinated verbs, and Danish often leaves it out in the second part.

So:

• ... tager jeg min regnjakke på og cykler med hjelm ...

means:

• ... I put on my rain jacket and cycle with a helmet ...

The subject jeg is understood for both verbs:

• tager
• cykler

You could say og jeg cykler, but it is less natural here unless you want extra emphasis.

Because here cykler is a verb form, not a plural noun.

• jeg cykler = I cycle / I ride a bike
• cykler as a noun would mean bicycles

The sentence structure makes it clear that it is a verb here:

• ... og cykler med hjelm ...

It follows og and continues the action of the sentence, so it must be verbal.

This is a very natural Danish way to say with a helmet on.

• med hjelm på literally = with helmet on

Here:

• med introduces what you have or are using
• på shows that the helmet is being worn

So:

• cykle med hjelm på = cycle wearing a helmet

English also often needs something similar:

• with a helmet on
• wearing a helmet

Danish often omits the article in this kind of expression, so med hjelm på sounds very natural.

This på is completely different from the på in tage ... på and the på in med hjelm på.

In på cykelstien, på is a normal preposition meaning on:

• på cykelstien = on the bike lane

So the sentence contains different kinds of på:

• tager ... på = particle verb, put on
• med hjelm på = with a helmet on
• på cykelstien = preposition, on the bike lane

They look the same, but they are doing different jobs.

Cykelstien is the definite form of cykelsti:

• en cykelsti = a bike lane
• cykelstien = the bike lane

Danish often uses the definite form when talking about the specific lane you are on or the relevant one in the situation.

So:

• på cykelstien = on the bike lane

If you said på en cykelsti, it would sound more like on a bike lane, meaning any one, not a specific or contextually understood one.

Danish often uses the present tense for things that are habitual, general, or future in context.

So:

• Hvis det regner, tager jeg min regnjakke på ...

can mean something like:

• If it rains, I put on my rain jacket ...
• or more naturally in English, If it rains, I’ll put on my rain jacket ...

The Danish present tense here works well because the sentence describes what the speaker does in that situation.

Because Hvis det regner is a subordinate clause placed before the main clause.

So the structure is:

• subordinate clause: Hvis det regner
• main clause: tager jeg min regnjakke på og cykler med hjelm på cykelstien

The comma helps separate those two parts. That is standard and very useful for reading the sentence clearly.