Hvis jeg mister lånekortet mitt, må jeg gå til skranken og be om hjelp med en gang.

Because hvis introduces a subordinate clause: Hvis jeg mister lånekortet mitt.

In Norwegian subordinate clauses, the usual word order is:

conjunction + subject + verb

So:

• Hvis jeg mister ... = If I lose ...

Not:

• Hvis mister jeg ... ❌

That second order would sound wrong in Norwegian.

This is because the sentence begins with the if-clause, and Norwegian follows the V2 rule in main clauses.

The structure is:

• Hvis jeg mister lånekortet mitt, må jeg gå til skranken ...

When something other than the subject comes first in the main sentence, the finite verb must come in second position, and the subject comes after it.

So:

• Hvis jeg mister lånekortet mitt, må jeg ...
• not Hvis jeg mister lånekortet mitt, jeg må ... ❌

Compare:

• Jeg må gå til skranken.
• Hvis jeg mister lånekortet mitt, må jeg gå til skranken.

In Norwegian, the present tense is very often used for future meaning or in if-clauses, just like in English.

So:

• Hvis jeg mister lånekortet mitt = If I lose my library card

Norwegian does not normally use a special future form here.

This is very similar to English:

• If I lose it, I have to ask for help.
• not usually If I will lose it ...

Miste is the infinitive form: to lose.

Mister is the present tense form: lose / am losing

Examples:

• å miste = to lose
• jeg mister = I lose / I’m losing

So in the sentence:

• Hvis jeg mister ... = If I lose ...

Both are possible in Norwegian, but they are used a little differently.

Most common everyday pattern

noun + definite ending + possessive

• lånekortet mitt

This is the most natural and common way to say my library card in ordinary speech.

Another possible pattern

possessive + noun

• mitt lånekort

This is also correct, but it can sound a bit more emphatic, contrastive, or formal depending on context.

So for a normal everyday sentence, lånekortet mitt is the most expected choice.

It is a compound noun:

• lån = loan / borrowing
• kort = card

So lånekort is a borrowing card, which in context usually means a library card.

Then -et is the definite ending for a neuter noun:

• et lånekort = a library card
• lånekortet = the library card

So:

• lånekortet mitt = my library card

Because til means to / toward, and it is used when there is movement toward a place.

• gå til skranken = go to the desk/counter

If you used i, that would usually suggest being in something, which does not fit well with skranken here.

For a service desk or counter, til skranken is the natural choice when you mean going over there.

Skranke usually means a counter, desk, or service desk.

In this sentence, it most likely means something like:

• the library desk
• the front desk
• the service counter

So gå til skranken means going to the place where staff can help you.

Be om is a fixed expression meaning ask for.

So:

• be om hjelp = ask for help

The word om is part of the expression. You should learn it as a unit:

• be om penger = ask for money
• be om råd = ask for advice
• be om hjelp = ask for help

If you leave out om, the meaning changes or the sentence may become incorrect in this context.

Because after a modal verb like må, Norwegian normally uses the bare infinitive.

Here:

• må gå
• må ... be

Not:

• må å gå ❌

So:

• jeg må gå = I must / have to go
• jeg må be om hjelp = I have to ask for help

In your sentence, both infinitives are connected:

• må jeg gå til skranken og be om hjelp

That is completely normal.

It can often be translated as either, depending on the context.

• må = must / have to

In this sentence, have to is probably the most natural English meaning:

• I have to go to the desk and ask for help right away

It expresses necessity or obligation.

Yes, literally it is something like with one time, but as an expression it means:

• immediately
• right away
• at once

So:

• be om hjelp med en gang = ask for help right away

It is a very common expression in Norwegian.

It can work as a general instruction or a likely situation, not necessarily something happening right now.

The pattern:

• Hvis ..., må ...

often gives the idea of:

• If X happens, then this is what I have to do
• or If X happens, this is the rule/procedure

So the sentence sounds natural for explaining what someone should do if they lose their library card.

Usually, hvis is the normal word for if in a conditional sentence like this.

• Hvis jeg mister lånekortet mitt ... = If I lose my library card ...

Using om for if is possible in some contexts, but it is not the most natural choice here. For ordinary conditionals, learners should normally use hvis.

So this sentence is best as:

• Hvis jeg mister lånekortet mitt, må jeg gå til skranken ...