Hvis regnen rammer forruden, tænder jeg vinduesviskeren.

Yes. Hvis is the standard Danish word for if in conditional sentences.

So:

• Hvis regnen rammer forruden ... = If the rain hits the windshield ...

It introduces a condition, just like English if.

Regnen is the definite form: the rain.

• regn = rain
• regnen = the rain

In Danish, weather or something already understood from the situation is often expressed with the definite form. Here, regnen refers to the rain that is falling and hitting the windshield.

A learner should also know that Danish often uses a different structure for weather:

• Hvis det regner ... = If it rains ...

That version is often more natural in everyday speech.
Hvis regnen rammer forruden ... is more concrete and visual: it focuses on the rain physically striking the windshield.

Because Danish, like English, often uses the present tense in conditional sentences when talking about something general, habitual, or future.

• rammer = hits / is hitting
• tænder = turn on / switch on

So this sentence means something like:

• If the rain hits the windshield, I turn on the windshield wiper
• or more naturally in English: If the rain hits the windshield, I switch on the wiper(s)

Even though the action may happen in the future, Danish does not need a future form here.

Because the sentence begins with a subordinate clause:

• Hvis regnen rammer forruden = subordinate clause
• tænder jeg vinduesviskeren = main clause

In standard Danish, a subordinate clause is separated from the main clause with a comma.

So the structure is:

• Hvis ... , ...

This is very common in Danish writing.

This is a very important Danish word-order rule.

Danish is a verb-second language in main clauses. That means the finite verb usually comes in the second position.

Here, the whole if-clause comes first:

• Hvis regnen rammer forruden, ...

Because that first part takes the first position, the verb in the main clause must come next:

• tænder jeg vinduesviskeren

So the order is:

1. initial element: Hvis regnen rammer forruden
2. finite verb: tænder
3. subject: jeg

If the main clause came first, you would say:

• Jeg tænder vinduesviskeren, hvis regnen rammer forruden.

Then the normal subject-verb order returns.

It comes from tænde, which often means to turn on, switch on, or light depending on context.

Examples:

• tænde lyset = turn on the light
• tænde fjernsynet = turn on the TV
• tænde en cigaret = light a cigarette

Here it means switch on:

• tænder jeg vinduesviskeren = I switch on the windshield wiper

So it is not about fire here; it is just the normal verb for activating something electrical or mechanical.

Good question. Danish can use the singular here, especially if it refers to the wiper mechanism as one system or one unit.

• vinduesviskeren = the windshield wiper
• vinduesviskerne = the windshield wipers

In real everyday language, many speakers would probably prefer the plural if they are thinking of the two blades:

• Hvis det regner, tænder jeg vinduesviskerne.

So the singular is understandable and grammatical, but the plural may sound more natural in many situations.

It is the definite singular form of forrude.

• forrude = windshield / front window of a car
• forruden = the windshield

It is an en-word in Danish, so the definite ending is -en.

Also, it is a compound noun:

• for- = front
• rude = pane / window

So forrude literally means something like front pane.

Danish uses compound nouns very freely, much like German.

forrude

• for = front
• rude = pane / window

So forrude = front window / windshield

vinduesvisker

• vindue = window
• visker = wiper / something that wipes

So vinduesvisker = window wiper

Then the definite form becomes:

• vinduesviskeren = the windshield wiper

Learning to recognize compounds is very useful in Danish, because many everyday nouns are built this way.

Sometimes, but the meaning changes slightly.

• hvis = if
• når = when

So:

• Hvis regnen rammer forruden ... = If the rain hits the windshield ...
• Når regnen rammer forruden ... = When the rain hits the windshield ...

Use hvis when the condition is possible or uncertain.
Use når when you present it as something that happens or will happen.

In many real-life contexts, especially repeated situations, når can sound very natural:

• Når det regner, tænder jeg vinduesviskerne. = When it rains, I turn on the wipers.

It is understandable, but many Danish speakers would probably say it a little differently in everyday speech.

A very natural version would be:

• Hvis det regner, tænder jeg vinduesviskerne.
• Når det regner, tænder jeg vinduesviskerne.

Why?

• det regner is the usual way to say it is raining / it rains
• vinduesviskerne in the plural often sounds more natural

Your original sentence is still useful for learning because it clearly shows:

• an if-clause
• Danish verb-second word order
• definite noun forms
• compound nouns

So it is a good teaching sentence, even if a native speaker might phrase it a bit differently in casual conversation.