Yksi matkustaja meni väärään terminaaliin ja joutui juoksemaan takaisin.

Both are possible, depending on context.

Finnish has no articles, so there is no direct equivalent of a/an or the. The word yksi literally means one, but it is also often used when introducing one person/one thing for the first time, where English would naturally say a.

So yksi matkustaja can mean:

• one passenger if the number is important
• a passenger if it is simply introducing someone into the story

In this sentence, it can easily feel like a passenger in English, even though Finnish uses yksi.

Because matkustaja is the subject of the sentence, and subjects are typically in the nominative in Finnish.

So here:

• yksi matkustaja = one/a passenger
• nominative singular, because this is the person doing the action

Also, the ending -ja is just part of the word matkustaja. It is not a case ending here.

Yes. Both are in the Finnish past tense.

• meni = went
  Dictionary form: mennä = to go
• joutui = had to / ended up having to
  Dictionary form: joutua

So the sentence describes completed events in the past.

A useful comparison:

• menee = goes

• meni = went

• joutuu = has to / ends up having to
• joutui = had to / ended up having to

Because Finnish marks direction with case endings, and adjectives must agree with the noun they describe.

Here the idea is into/to the wrong terminal, so Finnish uses the illative case, which often expresses movement into something.

Base forms:

• väärä terminaali = wrong terminal

Illative forms:

• väärään terminaaliin = into/to the wrong terminal

So:

• terminaaliin is the noun in the illative
• väärään is the adjective, also in the illative to match the noun

This kind of agreement is very important in Finnish.

Because the sentence describes movement to a place, not being in a place.

Compare:

• terminaaliin = into/to the terminal
• terminaalissa = in the terminal
• terminaalista = out of/from the terminal

Since the passenger went to the wrong terminal, Finnish uses the directional form terminaaliin.

Joutua often means something like:

• to end up having to
• to be forced by circumstances to
• to have to, unfortunately

So joutui juoksemaan takaisin is not just a neutral had to run back. It suggests that the passenger was in that situation because something had gone wrong.

That makes joutui a very natural choice here, because going to the wrong terminal caused an unwanted consequence.

A rough nuance comparison:

• piti juosta takaisin = had to run back
• joutui juoksemaan takaisin = ended up having to run back / was forced to run back

The second one sounds more like an unfortunate result.

Because after joutua, Finnish normally uses the third infinitive in the illative, which often ends in -maan / -mään.

So:

• juosta = to run (dictionary form)
• juoksemaan = the form used here after joutui

This structure is very common:

• joutui odottamaan = had to wait
• joutui maksamaan = had to pay
• joutui juoksemaan = had to run

So you should learn joutua + -maan/-mään form as a pattern.

Because Finnish often leaves out repeated pronouns when the subject is already clear.

In this sentence:

• Yksi matkustaja meni väärään terminaaliin
• ja joutui juoksemaan takaisin

The second verb joutui still refers to the same subject, yksi matkustaja.

English often repeats the subject more readily:

• A passenger went to the wrong terminal and had to run back.

Finnish does not need to repeat hän here. If you did add it, it would usually sound more emphatic than necessary.

Takaisin means back.

Here it tells us the direction of the running:

• juoksemaan takaisin = to run back

It comes at the end because that is a very natural place for it in Finnish. It modifies the action and completes the idea.

Finnish word order is fairly flexible, but this version is neutral and natural.

Not completely. Finnish word order is more flexible than English because case endings show grammatical relationships.

This sentence uses a very normal, neutral order:

• Yksi matkustaja = subject
• meni = verb
• väärään terminaaliin = destination
• ja joutui juoksemaan takaisin = second action

You could rearrange parts for emphasis, but the original sentence is the most straightforward version for normal narration.

For example, moving väärään terminaaliin earlier would emphasize the wrong destination more, but the given order is the most neutral.

Yes.

Using yksi matkustaja makes the sentence feel more like:

• one passenger
• a certain passenger
• an individual being introduced into the story

Without yksi, matkustaja meni väärään terminaaliin could sound less specific or depend more heavily on context.

So yksi helps present this as one particular passenger in the story, even if English translates it simply as a passenger.