Jos lähtöselvitys ei onnistu sovelluksessa, lentoyhtiö auttaa minua terminaalissa.

Jos means if. It introduces a condition:

• Jos lähtöselvitys ei onnistu sovelluksessa = If check-in does not work in the app
• lentoyhtiö auttaa minua terminaalissa = the airline helps me at the terminal

So the whole sentence has the pattern If X happens, Y happens.

Lähtöselvitys means check-in in the air-travel sense.

It is a compound word:

• lähtö = departure
• selvitys = clarification / processing / settlement

Finnish very often builds nouns this way, so one English phrase may become one long Finnish word.

Because Finnish commonly writes compound nouns as a single word. What English expresses as check-in, airline, boarding pass, and so on is often one written word in Finnish.

So:

• lähtöselvitys = check-in
• lentoyhtiö = airline

This is completely normal in Finnish.

In the Finnish present negative, the negative verb carries the person, and the main verb appears in a special form.

Compare:

• onnistuu = succeeds / works
• ei onnistu = does not succeed / does not work

Here:

• ei = negative verb for third person singular
• onnistu = the main verb in its negative form

So ei onnistuu is not correct Finnish.

It can mean both, depending on context.

Literally, onnistua means something like to succeed or to turn out successfully. In this sentence, natural English often uses work:

• Jos lähtöselvitys ei onnistu sovelluksessa
  = If check-in does not work in the app = If you cannot complete check-in in the app

So Finnish focuses on whether the process is successful, not necessarily on the person doing it.

The ending -ssa / -ssä is the inessive case, which usually means in.

So:

• sovellus = app / application
• sovelluksessa = in the app

Even though English often says on the app, Finnish normally says in the app with this case.

Usually sovelluksessa is the natural choice here.

• sovelluksessa = in the app
• sovelluksella = with the app / by means of the app

Because the idea is that check-in happens within the app interface, sovelluksessa fits best.

Finnish has no articles, so it does not have separate words for a, an, or the.

So:

• lentoyhtiö can mean an airline or the airline
• the exact meaning comes from context

In this sentence, English naturally uses the airline, but Finnish does not need a special word for that.

Because the verb auttaa normally takes the partitive object.

So:

• minä = I
• minua = me, in the partitive form

With auttaa, Finnish says:

• auttaa minua = help me
• auttaa häntä = help him/her
• auttaa meitä = help us

This is something you mostly learn as part of the verb’s pattern: auttaa jotakuta = to help someone.

Again, -ssa / -ssä means in.

• terminaali = terminal
• terminaalissa = in the terminal

So the sentence says the help will be given in the terminal. English might sometimes say at the terminal, but Finnish uses in here.

Yes. Finnish often uses the present tense for future meaning when the context makes it clear.

So:

• ei onnistu = does not work / cannot be completed
• auttaa = helps / will help

In this sentence, the meaning is future-oriented because it describes what will happen if a certain situation occurs.

Yes. Finnish word order is fairly flexible.

For example, you could also say:

• Lentoyhtiö auttaa minua terminaalissa, jos lähtöselvitys ei onnistu sovelluksessa.

That means the same thing. Starting with the jos clause just puts the condition first, which is very natural.

Yes. In the clause lähtöselvitys ei onnistu sovelluksessa, the subject is lähtöselvitys.

That is why the verb is in third person singular:

• lähtöselvitys onnistuu
• lähtöselvitys ei onnistu

Finnish is literally saying the check-in does not succeed/work in the app.

Yes. Finnish could express that idea more personally, for example with a structure meaning if I can’t do the check-in in the app.

But the version in your sentence is very natural because Finnish often describes whether a process works or succeeds, rather than focusing on the person. It sounds neutral and typical of customer-service language.