Passintarkastus sujuu nopeasti, mutta yksi matkustaja joutuu palaamaan takaisin turvatarkastukseen.

Because sujuu is a finite verb form: it is the 3rd person singular present tense of sujua.

• sujua = to go smoothly, to proceed, to run
• sujuu = goes smoothly / proceeds

The subject is passintarkastus (passport control), which is singular, so the verb is also singular:

• Passintarkastus sujuu nopeasti. = Passport control goes quickly / runs smoothly.

So:

• dictionary form: sujua
• sentence form: sujuu

Because nopeasti is an adverb, while nopea is an adjective.

• nopea = fast, quick
• nopeasti = quickly, fast

Here the word describes how the action happens, not what kind of thing something is:

• Passintarkastus sujuu nopeasti. = passport control proceeds quickly

Compare:

• nopea tarkastus = a quick inspection
• tarkastus sujuu nopeasti = the inspection goes quickly

A very common Finnish pattern is:

• adjective stem + -sti → adverb

For example:

• hidas → hitaasti = slow → slowly
• varma → varmasti = sure/certain → certainly

In this sentence, sujuu means something like:

• goes smoothly
• proceeds well
• moves along without problems

So Passintarkastus sujuu nopeasti does not just mean that passport control is fast in an abstract sense. It suggests that the process is running smoothly and quickly.

This verb is often used for processes, events, work, conversations, studies, and so on:

• Työ sujuu hyvin. = The work is going well.
• Matka sujui ongelmitta. = The trip went without problems.
• Keskustelu sujui helposti. = The conversation went easily.

Because yksi means one, and with yksi, the noun is in the singular.

• yksi matkustaja = one passenger

This is different from numbers greater than one:

• yksi matkustaja = one passenger
• kaksi matkustajaa = two passengers
• kolme matkustajaa = three passengers

So a learner often notices that with kaksi, kolme, neljä..., Finnish uses the partitive singular for the noun, but yksi behaves differently and takes the normal singular form.

Joutuu is the 3rd person singular of joutua.

In this sentence, joutua means:

• to have to
• to be forced to
• to end up having to

So:

• yksi matkustaja joutuu palaamaan takaisin turvatarkastukseen = one passenger has to go back to security or = one passenger ends up having to return to security

A useful nuance: joutua often suggests that the situation is not voluntary or is caused by circumstances.

Compare:

• Hän palaa turvatarkastukseen. = He/she returns to security.
  (neutral statement)
• Hän joutuu palaamaan turvatarkastukseen. = He/she has to return to security.
  (something makes this necessary)

Because joutua is commonly followed by the third infinitive illative, which has the form -maan / -mään.

So:

• palata = to return
• palaamaan = into the act of returning / to return

After joutua, Finnish normally uses this structure:

• joutua + verb-maan/-mään

Examples:

• joutuu odottamaan = has to wait
• joutuu lähtemään = has to leave
• joutuu palaamaan = has to return

This is a very common pattern, and it is best learned as a set:

• joutua tekemään jotakin = to have to do something

So in this sentence:

• yksi matkustaja joutuu palaamaan = one passenger has to return

Yes, it is a little repetitive in a literal sense, because palata already means to return / go back, and takaisin also means back.

But in real Finnish, this is very natural. It adds emphasis and sounds idiomatic.

• palata = return
• takaisin = back

So:

• palata turvatarkastukseen = return to security
• palata takaisin turvatarkastukseen = go back to security

This kind of doubling happens in many languages. In Finnish, palata takaisin is common and not considered strange in everyday usage.

Because turvatarkastukseen is in the illative case, which often expresses movement into or to something.

The basic form is:

• turvatarkastus = security check / security screening

The illative form is:

• turvatarkastukseen = into/to the security check

Since the passenger is going back to security screening, Finnish uses the case of direction:

• palaa turvatarkastukseen = returns to security

Compare:

• turvatarkastuksessa = in/at security
• turvatarkastuksesta = from security
• turvatarkastukseen = into/to security

So the ending tells you the direction of movement.

Yes. Finnish forms compound words very often.

Here:

• passi = passport
• tarkastus = inspection, check

So:

• passintarkastus = passport check / passport control

And:

• turva = safety, security
• tarkastus = inspection, check

So:

• turvatarkastus = security check / security screening

Notice that in passintarkastus, the first part appears as passin-, which is the genitive form of passi:

• passi → passin
• passin + tarkastus → passintarkastus

This is a very common way to build Finnish compounds.

Because Finnish has no articles.

So Finnish does not have separate words corresponding directly to English a/an and the.

That means:

• Passintarkastus can mean passport control or the passport control, depending on context.
• yksi matkustaja clearly means one passenger, because yksi supplies the meaning.
• turvatarkastukseen can mean to security, to the security check, or into the security screening area, depending on context.

English needs articles, but Finnish usually lets the context carry that information.

That is the normal Finnish word order for a basic statement:

• subject + verb

So:

• yksi matkustaja = subject
• joutuu = verb

Finnish word order is flexible, but the neutral order here is very natural:

• mutta yksi matkustaja joutuu palaamaan...

You could change the order in some contexts for emphasis, but this version is the most straightforward.

The whole sentence is built from two clauses joined by mutta = but:

1. Passintarkastus sujuu nopeasti
2. mutta yksi matkustaja joutuu palaamaan takaisin turvatarkastukseen

So the second clause simply starts with but one passenger...

Yes. Mutta is the ordinary coordinating conjunction meaning but.

It links two contrasting ideas:

• passport control is going smoothly and quickly
• however, one passenger has to go back to security

So the contrast is:

• overall things are moving well
• but there is one exception/problem

This is exactly the kind of situation where Finnish uses mutta.

A useful breakdown is:

• Passintarkastus = passport control
• sujuu = goes smoothly / proceeds
• nopeasti = quickly
• mutta = but
• yksi = one
• matkustaja = passenger
• joutuu = has to / ends up having to
• palaamaan = to return
• takaisin = back
• turvatarkastukseen = to security screening

A more natural English-style reconstruction would be:

• Passport control is going quickly, but one passenger has to go back to security screening.

Or more literally:

• Passport control proceeds quickly, but one passenger is forced to return back to the security check.