Settu bílbeltið á þig áður en við förum.

Settu is the singular imperative of setja (to put, place, set).

So the sentence is directed at one person and means something like a command or instruction: Put on ...

Compare:

• setja = to put
• settu = put! (said to one person)
• setjið = put! (said to more than one person, or used politely)

Icelandic often does not use a possessive like your when ownership is already obvious from the context.

Here, á þig (on you) makes it clear whose seat belt is meant, so bílbeltið literally means the seat belt, but in natural English we often translate it as your seat belt.

This is very common with things connected to the body, clothing, and personal items when the owner is obvious.

Bílbeltið breaks down like this:

• bíll = car
• belti = belt
• bílbelti = seat belt / car belt
• bílbeltið = the seat belt

The -ð at the end is the suffixed definite article (the).

Also, bílbeltið is the direct object of settu. Since belti is a neuter noun, its singular nominative and accusative forms are the same, so you do not see any case change here.

Because Icelandic uses setja ... á ... for putting something onto someone/something.

So:

• setja bílbeltið á þig = put the seat belt on you / put on the seat belt

The preposition á here expresses movement or placement onto the person.

Because þig is the accusative form of þú.

After the preposition á in this kind of movement/placement expression, Icelandic uses the accusative, so:

• þú = you (subject form)
• þig = you (object form)

That is why á þig is correct, not á þú.

Not in this sentence.

• á þig = on you
• á sig = on oneself / himself / herself / themselves, depending on context

Since the speaker is talking to another person, Icelandic uses þig.

Examples:

• Settu bílbeltið á þig. = Put your seat belt on.
• Hann setti bílbeltið á sig. = He put the seat belt on himself.

Áður en means before.

It introduces a subordinate clause:

• áður en við förum = before we go / before we leave

So the structure is:

• Settu bílbeltið á þig = Put on the seat belt
• áður en við förum = before we leave

Together: Put on your seat belt before we leave.

Because Icelandic often uses the present tense to talk about the future when the context already makes the time clear.

So:

• við förum literally = we go
• but naturally here = we leave / we’re leaving

English does this sometimes too:

• We leave in five minutes.

So this is completely normal Icelandic.

Because áður en introduces a subordinate clause, and in subordinate clauses Icelandic usually has subject + verb order:

• áður en við förum

In a main clause, Icelandic often follows V2 word order, but subordinate clauses do not work the same way.

So after áður en, við förum is the expected order.

It is correct and perfectly understandable.

That said, in everyday Icelandic, people also very often use verbs meaning fasten or buckle, for example:

• Spenntu bílbeltið.
• Spenntu beltið.

Those can sound especially natural for Buckle your seat belt / Fasten your seat belt.

So the given sentence is good Icelandic, but it is useful to know that spenna beltið is also very common.

Yes, Icelandic allows some flexibility in word order, and Settu á þig bílbeltið can also be understood.

However, Settu bílbeltið á þig is a very straightforward and natural order:

• verb
• object
• prepositional phrase

A different order can shift emphasis slightly, but the basic meaning stays the same.