Hvis jeg er i tvivl om adressen, tjekker jeg kortet igen.

Because Danish uses verb-second word order in main clauses.

The first part, Hvis jeg er i tvivl om adressen, is a subordinate clause beginning with hvis (if). When that whole clause comes first, the finite verb in the main clause must come next:

• Hvis jeg er i tvivl om adressen, tjekker jeg kortet igen.

So the structure is:

• subordinate clause
• verb
• subject

Compare:

• Jeg tjekker kortet igen, hvis jeg er i tvivl om adressen.
• Hvis jeg er i tvivl om adressen, tjekker jeg kortet igen.

Both are correct, but the word order changes when the if-clause comes first.

I tvivl is a fixed expression meaning in doubt / unsure.

So:

• jeg er i tvivl = I am in doubt / I am unsure

Danish often uses combinations like verb + prepositional phrase where English might use a single adjective. In natural English, you would usually say I’m unsure or I’m not sure, but Danish commonly says:

• at være i tvivl = to be in doubt / to be unsure

It is best learned as a whole expression.

Because Danish uses om after i tvivl to introduce what you are unsure about.

• i tvivl om noget = in doubt about something / unsure about something

So:

• Jeg er i tvivl om adressen = I’m unsure about the address

More examples:

• Hun er i tvivl om svaret. = She is unsure about the answer.
• Vi er i tvivl om tidspunktet. = We are unsure about the time.

So om is just the normal preposition used with this expression.

Because adressen is the definite form: the address.

In Danish, nouns usually add an ending to show definiteness:

• en adresse = an address
• adressen = the address

Here, the speaker means a specific address already known from the situation, so Danish uses the definite form.

This is very common in Danish, where definiteness is often built into the noun itself instead of using a separate word like the.

Tjekker is the present tense of at tjekke (to check).

Forms:

• at tjekke = to check
• tjekker = check / am checking / checks
• tjekkede = checked
• har tjekket = have checked

In this sentence, the present tense is used for a general habit or repeated action:

• Hvis jeg er i tvivl ..., tjekker jeg ...
• If I’m unsure ..., I check ...

So it is not necessarily happening right now; it can describe what the speaker normally does.

Kortet is the definite form of kort.

• et kort = a map / a card
• kortet = the map / the card

In this sentence, kortet means the map, not the card.

Danish kort can mean different things depending on context:

• et kort = a map
• et kort = a card
• et kort = a short one / something short (in other contexts as an adjective-derived noun)

Here, because the sentence is about finding an address, kortet clearly means the map.

Usually yes. Igen means again, and in this sentence it means the speaker checks the map one more time.

• tjekker jeg kortet igen = I check the map again

Its placement is also natural here at the end of the clause.

You may also see igen in slightly different positions, but sentence-final position is very common and easy to use.

It is most naturally understood as a general condition or habitual situation.

• Hvis jeg er i tvivl om adressen, tjekker jeg kortet igen.
• If I’m unsure about the address, I check the map again.

Both Danish and English often use the present tense for general truths, habits, or regular reactions.

So this sentence does not have to mean only right now. It often means whenever this happens, that is what I do.

Yes, that is possible.

• Jeg er i tvivl om adressen = I am unsure / in doubt about the address
• Jeg er usikker på adressen = I am unsure about the address

Both are natural, but there is a slight difference in feel:

• i tvivl om often emphasizes doubt or uncertainty about whether something is correct
• usikker på often emphasizes being not sure or lacking confidence

Also notice the preposition changes:

• i tvivl om
• usikker på

So you cannot freely swap only the adjective and keep the same preposition.

Because the sentence begins with a subordinate clause:

• Hvis jeg er i tvivl om adressen = if I am unsure about the address

Then comes the main clause:

• tjekker jeg kortet igen

In standard Danish writing, a comma is commonly used to separate these parts. So the comma helps show where the if-clause ends and the main clause begins.

A careful pronunciation might be something like:

• vis yai air i tviwl om ah-DRES-sen, CHEK-er yai KOR-det i-GEN

A few useful notes:

• Hvis is often pronounced close to vis
• jeg is often pronounced something like yai in careful speech, though real pronunciation can vary
• tvivl can be tricky because of the consonant cluster
• tjekker starts with a ch-like sound, similar to English check
• igen is often stressed on the second syllable: i-GEN

As in all languages, everyday speech may sound more reduced than textbook pronunciation.

Yes:

• Jeg tjekker kortet igen, hvis jeg er i tvivl om adressen.

This means the same thing.

The difference is mainly in emphasis and word order:

• Starting with Hvis ... emphasizes the condition first
• Starting with Jeg tjekker ... emphasizes the action first

Notice that when the main clause comes first, normal order returns:

• Jeg tjekker not
• tjekker jeg

So the inversion only happens because the sentence begins with another element before the main clause.