Um leið og ég sest í bílinn, set ég bílbeltið á mig.

Um leið og is a fixed expression meaning as soon as, when, or just as.

In this sentence, it introduces the time clause:

Um leið og ég sest í bílinn
= As soon as I get into the car

Literally, um leið is something like at the same time / in the same moment, so the whole phrase gives the idea of one action happening immediately when another begins.

Because sest and sit mean different things.

• ég sit = I am sitting
• ég sest = I sit down / I take a seat / I get seated

Here the sentence is about the action of getting into the car and taking your seat, not the state of already sitting there.

So:

• Ég sit í bílnum = I am sitting in the car
• Ég sest í bílinn = I get into the car and sit down

The verb setjast is the verb used for sit down / take a seat.

They are closely related, but they are used differently:

• setja = to put, place, set something
• setjast = to sit down, seat oneself

So in your sentence:

• ég sest í bílinn uses setjast: I sit down / get seated in the car
• set ég bílbeltið á mig uses setja: I put the seat belt on myself

This is a nice example of how Icelandic often uses an -st form to make a verb more reflexive or intransitive.

Because í can take either the accusative or the dative, depending on meaning.

• í + accusative = motion into
• í + dative = location in

So:

• í bílinn = into the car
• í bílnum = in the car

Since the sentence describes movement into the car, Icelandic uses the accusative:

ég sest í bílinn = I get into the car / I sit down in the car

If you were just describing where you already are, you would say:

Ég sit í bílnum.
= I am sitting in the car.

This is because Icelandic follows a verb-second pattern in main clauses.

If the sentence were standing alone, you would normally say:

Ég set bílbeltið á mig.

But here the sentence begins with a time clause:

Um leið og ég sest í bílinn, ...

That whole clause takes the first position, so in the main clause the finite verb comes next:

..., set ég bílbeltið á mig.

So the word order is:

1. fronted element: Um leið og ég sest í bílinn
2. finite verb: set
3. subject: ég

This is very common in Icelandic.

They show that Icelandic usually puts the at the end of the noun, not as a separate word.

• bíll = car

• bílinn = the car

• bílbelti = seat belt
• bílbeltið = the seat belt

In this sentence:

• bílinn is the accusative singular definite form of bíll
• bílbeltið is the accusative singular definite form of bílbelti

So instead of saying a separate word for the, Icelandic attaches it to the noun.

Because Icelandic often uses setja ... á mig/þig/hann/sig for put ... on me/you/him/oneself.

So:

set ég bílbeltið á mig
literally = I put the seat belt onto me

That is the Icelandic way of expressing I put on the seat belt.

Also, mig is in the accusative, because á often takes:

• accusative for movement onto something
• dative for location on something

Compare:

• Ég set bílbeltið á mig. = I put the seat belt on myself.
• Bílbeltið er á mér. = The seat belt is on me.

So á mig fits the idea of an action resulting in something being placed onto your body.

Yes, very often it can.

You may hear both:

• set ég bílbeltið á mig
• set ég á mig bílbeltið

Both are understandable and natural. The difference is mostly about phrasing, rhythm, and emphasis.

Learners will often meet this pattern with clothing too:

• Ég set á mig húfu. = I put on a hat.
• Ég fer í skóna. = I put on shoes.

So if you see á mig before or after the noun, that is not surprising.

Most naturally, it sounds habitual:

Whenever / As soon as I get into the car, I put on my seat belt.

Icelandic often uses the present tense for actions that are generally true, routine, or repeated.

So this sentence is best understood as describing a regular habit, not necessarily one single event happening at this exact moment.

No. It is a good sentence, but Icelandic has other natural ways to express the same idea.

A very common verb is spenna when talking about fastening a seat belt:

• Um leið og ég sest í bílinn, spenni ég bílbeltið.
• Um leið og ég sest í bílinn, spenni ég mig í beltið. (different structure, also possible in some contexts)

Your original sentence is perfectly understandable, but learners should know that Icelanders often use spenna bílbeltið for fasten the seat belt.

A helpful rough breakdown is:

Um leið og = as soon as / just as
ég sest = I sit down / I get seated
í bílinn = into the car
set ég = I put
bílbeltið = the seat belt
á mig = on me / onto myself

So very literally, it is something like:

As soon as I seat myself into the car, I put the seat belt onto myself.

That is not natural English, but it helps show how the Icelandic sentence is built.