Teen lähtöselvityksen kotona ennen kuin menen lentokentälle.

In Finnish, the verb ending usually tells you who the subject is.

• teen = I do / I make
• menen = I go

The ending -n marks first person singular: I.

So minä is often left out because it is already clear from the verb. You can say Minä teen lähtöselvityksen..., but that sounds more emphatic, like I do the check-in...

Because tehdä is an irregular verb.

Its present-tense forms use the stem tee-:

• teen = I do
• teet = you do
• tekee = he/she does
• teemme = we do
• teette = you all do
• tekevät = they do

So teen is just the correct present-tense form of tehdä.

Lähtöselvitys is the standard Finnish word for check-in in air travel.

Literally, it is made from:

• lähtö = departure
• selvitys = clarification / processing / sorting out

But in normal usage, tehdä lähtöselvitys simply means to check in for a flight.

So Teen lähtöselvityksen means I check in or more literally I do the check-in.

Because here it is the object of the verb teen.

In this sentence, the speaker is doing the whole action of check-in, so Finnish uses the total object. In the singular, that usually appears in the -n form:

• lähtöselvitys = check-in
• lähtöselvityksen = the whole check-in, as the completed object

So:

• Teen lähtöselvityksen = I complete the check-in

A useful comparison:

• En tee lähtöselvitystä = I am not doing the check-in

In the negative, Finnish uses the partitive instead.

Kotona means at home.

This is one of the special location forms used with koti:

• kotona = at home
• kotoa = from home
• kotiin = home / to home

These are very common and should mostly be learned as a set.

A learner might expect something based directly on koti, but kotona is the normal idiomatic word for at home.

There is also kodissa, but that usually sounds more like in the house/home building itself, while kotona is the normal way to say at home in everyday Finnish.

Because ennen kuin means before when it is followed by a full clause with a finite verb.

Here the clause is:

• ennen kuin menen lentokentälle = before I go to the airport

So the structure is:

• ennen kuin
  • verb clause

You cannot use just ennen by itself in the same way here.

A different structure is also possible:

• ennen lentokentälle menoa = before going to the airport

But in your sentence, ennen kuin menen lentokentälle is a very natural choice.

Finnish usually does not have a separate future tense.

Instead, it uses the present tense and lets the context show that the action is future.

So:

• Teen lähtöselvityksen kotona ennen kuin menen lentokentälle.

literally looks like:

• I do the check-in at home before I go to the airport.

But naturally in English, it means:

• I’ll check in at home before I go to the airport
• or I do the check-in at home before going to the airport, depending on context.

This is completely normal in Finnish.

Because -lle is the ending used here for movement to a place.

• lentokenttä = airport
• lentokentälle = to the airport

This is the allative form.

Compare:

• lentokentällä = at the airport
• lentokentältä = from the airport

So:

• menen lentokentälle = I go to the airport

That is the standard and natural expression.

Yes, you could, but the nuance changes slightly.

• mennä focuses on going to the destination
• lähteä focuses on leaving / setting off

So:

• ennen kuin menen lentokentälle = before I go to the airport
• ennen kuin lähden lentokentälle = before I leave for the airport

Both can be natural. In your sentence, menen is a straightforward, neutral choice.

Not completely. Finnish word order is fairly flexible, although this sentence uses a very neutral order:

• Teen lähtöselvityksen kotona ennen kuin menen lentokentälle.

That is a normal, unmarked way to say it.

You could also move things for emphasis, for example:

• Kotona teen lähtöselvityksen ennen kuin menen lentokentälle.

This puts more focus on at home.

So the original word order is not the only possible one, but it is a very natural default.

Because Finnish has no articles.

So Finnish does not have direct equivalents of English a/an and the in most situations.

That means:

• lentokentälle can mean to an airport or to the airport
• lähtöselvityksen can mean something like the check-in or just check-in

The exact meaning comes from the context. In this sentence, English naturally uses the airport, because a specific airport is understood.