Jag måste be om ursäkt för att jag kom sent.

Because måste is a modal verb, and in Swedish modal verbs are followed by the bare infinitive — that is, the infinitive without att.

So:

• Jag måste be om ursäkt = I must apologize
• not Jag måste att be om ursäkt

Other common modal verbs work the same way:

• Jag kan komma = I can come
• Du vill gå = You want to go
• Vi ska äta = We will/are going to eat

So after måste, just use the verb directly: be.

Literally, be om ursäkt means ask for forgiveness/excuse.

Breakdown:

• be = ask
• om = for
• ursäkt = excuse / forgiveness / apology

As a full expression, be om ursäkt means to apologize.

So even though English usually uses one verb, apologize, Swedish often uses this multi-word expression instead.

Examples:

• Jag vill be om ursäkt. = I want to apologize.
• Han bad om ursäkt. = He apologized.

Because be om ursäkt is a fixed expression. The verb be often takes om when it means ask for something.

Compare:

• be om hjälp = ask for help
• be om pengar = ask for money
• be om ursäkt = ask for forgiveness / apologize

So om is not optional here. It belongs to the expression.

Here, för att introduces what the apology is for.

So:

• Jag måste be om ursäkt för att jag kom sent. = I have to apologize for the fact that I arrived late.

After be om ursäkt för, Swedish can continue with:

• a noun: för misstaget = for the mistake
• an infinitive clause: för att vara sen = for being late
• a full clause: för att jag kom sent = for the fact that I arrived late

Be careful: för att can also mean in order to, but not here. In this sentence it means something more like for the fact that / because.

Kom is the preterite (simple past) of komma.

Swedish often uses the simple past where English might use either:

• I came late
• I arrived late
• sometimes even I have come late, depending on context

In this sentence, jag kom sent sounds very natural because it refers to a completed event in the past: you arrived late.

So:

• jag kom sent = I arrived late
• jag har kommit sent is grammatically possible in some contexts, but it is less natural here

Yes, often you could, but the nuance is a little different.

• jag kom sent = I arrived late
• jag var sen = I was late

So kom sent focuses more on the arrival, while var sen focuses more on the state of being late.

Both can work in apology contexts:

• Jag måste be om ursäkt för att jag kom sent.
• Jag måste be om ursäkt för att jag var sen.

The first one is a bit more specific about arriving late.

Because sent is the form used here as an adverb.

The word sen can be an adjective:

• Jag är sen. = I am late.

But when it describes how someone came/arrived, Swedish uses the adverb form:

• Han kom sent. = He arrived late.
• Vi gick sent. = We left late.

So:

• sen = adjective form
• sent = adverb form

That is why the sentence has kom sent, not kom sen.

Because after att, this is a subordinate clause, and Swedish subordinate clauses normally keep subject before verb.

So the normal order is:

• att jag kom sent
• because/that I arrived late

This is different from main clauses, where Swedish often uses verb-second word order:

• Jag kom sent. = main clause
• I går kom jag sent. = the verb kom still comes early because of main-clause word order

But after att, the clause is subordinate, so the order stays more straightforward:

• ... att jag kom sent

Måste is the present-tense form of måste/måsta in modern usage, but it often corresponds to English must or have to.

So:

• Jag måste be om ursäkt = I must apologize / I have to apologize

Even though måste is present tense, the apology can be about something that happened in the past:

• måste = the obligation exists now
• kom = the lateness happened in the past

So the sentence combines:

• a present obligation
• with a past event

Yes, it sounds a bit more formal or deliberate than just saying Förlåt or ursäkta.

Compare:

• Förlåt! = Sorry!
• Ursäkta! = Excuse me / Sorry!
• Jag måste be om ursäkt. = I must apologize.

So this sentence sounds appropriate when you want to sound more careful, polite, or serious about the apology.

No, not in this sentence.

You say:

• be om ursäkt för något
• be om ursäkt för att ...

The för belongs with ursäkt here: you apologize for something.

So:

• Jag måste be om ursäkt för att jag kom sent. = correct
• Jag måste be om ursäkt att jag kom sent. = not correct in standard Swedish

Think of it as the pattern:

• apologize for X
• Swedish: be om ursäkt för X