Við ætlum að láta athuga bílinn áður en við förum í ferðalag.

Because ætla normally takes an infinitive with að.

So the pattern is:

ætla að + infinitive

Examples:

• Ég ætla að fara = I intend to go
• Við ætlum að láta... = We intend/plan to have...

Here, ætlum is the 1st person plural present form of ætla: we intend / we are going to.

Because láta works differently from ætla here.

In this sentence, láta is being used in a causative sense: to have something done / make someone do something / let someone do something.

The structure is:

láta + infinitive

So:

• láta athuga = have (someone) check

Not:

• láta að athuga

This is very common in Icelandic:

• láta laga bílinn = have the car repaired
• láta þrífa húsið = have the house cleaned

It means to have the car checked.

Literally, láta can mean let, make, or cause, depending on context. In this sentence, it is best understood as arrange for someone to check the car.

So the idea is not necessarily that we ourselves will inspect the car. It usually suggests that someone else, such as a mechanic, will do it.

If you wanted to make the agent explicit, you could say something like:

• Við ætlum að láta verkstæði athuga bílinn = We plan to have a garage/workshop check the car

Because bílinn is the accusative form, and the verb athuga takes a direct object.

• bíllinn = the car in the nominative case
• bílinn = the car in the accusative case

Since the car is the thing being checked, it is the object of athuga, so Icelandic uses the accusative:

• að athuga bílinn = to check the car

This kind of case change is very normal in Icelandic nouns.

Because Icelandic usually puts the definite article at the end of the noun rather than using a separate word like English the.

So:

• bíll = a car
• bíllinn / bílinn = the car

The exact form changes with case:

• bíllinn = nominative
• bílinn = accusative

So in this sentence, bílinn means the car, with the article built into the word.

Because the clause after áður en is its own full clause, and it needs its own subject.

So Icelandic says:

• áður en við förum = before we go

Even though við already appeared earlier, Icelandic normally does not leave it out here.

Structure:

• main clause: Við ætlum að láta athuga bílinn
• subordinate clause: áður en við förum í ferðalag

Each clause has its own subject and verb.

Yes. Icelandic often uses the present tense where English would use future after time words like before, when, after, and so on.

So:

• áður en við förum literally looks like before we go
• but in context it means before we go / before we leave in the future

That is completely normal.

There is also a useful extra point: after áður en, Icelandic often uses the subjunctive when the event has not happened yet. With við, the present subjunctive and present indicative of fara happen to look the same: förum. So learners often cannot see the distinction in this exact sentence.

Because this is a subordinate clause, and subordinate clauses in Icelandic normally do not use the same verb-second pattern as main clauses.

Main clause:

• Við ætlum...

Subordinate clause after áður en:

• áður en við förum...

So the normal order here is: conjunction + subject + verb

That is why við förum is the natural order.

Fara simply means to go.

But fara í ferðalag is a set expression meaning to go on a trip / go traveling / set off on a journey.

So ferðalag adds the idea that this is not just ordinary movement from one place to another; it is a trip or journey.

Compared with ferð, ferðalag often sounds a bit more like a journey or excursion, sometimes a somewhat longer or more purposeful trip.

Because it is being used in a general, indefinite sense: go on a trip.

So:

• í ferðalag = on a trip / on a journey

This is a very natural expression in Icelandic:

• fara í ferðalag

You do not need the definite article unless you are talking about a specific known trip in context.

Yes, but the meaning changes.

• Við ætlum að athuga bílinn... = We plan to check the car ourselves
• Við ætlum að láta athuga bílinn... = We plan to have the car checked

So the version with láta suggests arranging for the check to be done, often by someone else.

That distinction is very important:

• athuga bílinn = check the car
• láta athuga bílinn = have the car checked

Yes, athuga works well and means check / inspect / examine.

In everyday Icelandic, you may also hear:

• skoða bílinn = inspect/look over the car
• láta skoða bílinn = have the car inspected

Both are natural, though skoða can sometimes sound a bit more like a physical inspection, while athuga is a broad check/examine verb. In this sentence, athuga is perfectly normal.