Við létum lyklasmið skipta um lásinn áður en við fórum í frí.

Létum is the 1st person plural past tense of láta.

In this kind of sentence, láta + person/object + infinitive often means to have someone do something or to get something done.

So:

• Við létum lyklasmið skipta um lásinn
  = We had a locksmith replace the lock

It does not usually mean that we physically forced the locksmith. In everyday language, it normally means we arranged for it to be done.

Because lyklasmið is the accusative singular form of lyklasmiður.

The dictionary form is:

• lyklasmiður = locksmith

But after láta in this construction, the person who is made/asked/hired to do the action appears in the accusative:

• Við létum lyklasmið skipta um lásinn

So this is a very common Icelandic pattern:

• láta einhvern gera eitthvað
  = make/have someone do something

Here:

• einhvern is accusative
• so lyklasmiður becomes lyklasmið

Because after láta in this construction, Icelandic uses the bare infinitive.

So you get:

• láta einhvern gera eitthvað
• not láta einhvern að gera eitthvað

That is why the sentence has:

• létum lyklasmið skipta um lásinn

and not:

• létum lyklasmið að skipta um lásinn

This is similar to English make/have someone do something, where English also often avoids to:

• We had the locksmith replace the lock
• not We had the locksmith to replace the lock

Skipta um means to change, replace, or switch out something.

So:

• skipta um lásinn = replace the lock

This is a fixed verb phrase you should learn as a unit:

• skipta um dekk = change a tire / tires
• skipta um föt = change clothes
• skipta um lás = change a lock

So in this sentence, skipta by itself is not the full idea; skipta um is the important expression.

Lásinn means the lock.

The base noun is:

• lás = lock

The ending -inn is the definite article attached to the noun, so:

• lás = a lock
• lásinn = the lock

It is also the correct case form here, because skipta um takes the accusative, and lásinn is the accusative singular definite form.

So:

• skipta um lás = replace a lock
• skipta um lásinn = replace the lock

The structure is:

• Við létum = we had / we got
• lyklasmið = a locksmith
• skipta um lásinn = replace the lock

So the pattern is:

• subject + láta (past) + person in accusative + infinitive phrase

More abstractly:

• Við létum [lyklasmið] [skipta um lásinn].

That whole part means:

• We had a locksmith replace the lock.

This is a very useful Icelandic structure.

Áður en means before.

It introduces a subordinate clause:

• áður en við fórum í frí
• before we went on vacation

So the sentence has two parts:

1. Við létum lyklasmið skipta um lásinn
2. áður en við fórum í frí

Literally:

• We had a locksmith replace the lock before we went on vacation.

This is a common conjunction pair in Icelandic, and you can think of áður en as a set phrase.

Fórum is the 1st person plural past tense of fara = to go.

So:

• við fórum = we went

That matches the subject við = we.

The sentence is talking about a completed event in the past, so past tense is exactly what you would expect:

• áður en við fórum í frí
  = before we went on vacation

Fara í frí is an idiomatic expression meaning to go on vacation / go on holiday.

So:

• í frí does not mean you are literally going physically into a thing called vacation
• it is just the standard Icelandic way to express entering that state

Common related expressions are:

• fara í frí = go on vacation
• vera í fríi = be on vacation / be off work
• koma úr fríi = come back from vacation

So this phrase is best learned as a chunk.

Without a definite article, lyklasmið here usually means a locksmith.

Icelandic often leaves indefinite nouns without any article at all:

• lyklasmiður = a locksmith
• lyklasmiðurinn = the locksmith

In this sentence, the accusative form is used:

• lyklasmið = a locksmith
• lyklasmiðinn = the locksmith

So the sentence most naturally means:

• We had a locksmith replace the lock

If the speaker meant a specific known locksmith, they could say:

• Við létum lyklasmiðinn skipta um lásinn

Yes. That is very natural Icelandic.

• Við létum skipta um lásinn
  = We had the lock replaced

This version leaves out the person doing the action. That is useful when the identity of the worker is unimportant or obvious from context.

So the difference is:

• Við létum lyklasmið skipta um lásinn
  = We had a locksmith replace the lock.
• Við létum skipta um lásinn
  = We had the lock replaced.

Both are good, but the first one specifically names who did the work.

A fairly literal breakdown is:

• Við = we
• létum = had / got
• lyklasmið = a locksmith
• skipta um = replace / change
• lásinn = the lock
• áður en = before
• við fórum = we went
• í frí = on vacation

So the sentence is built very much like this:

• We had a locksmith replace the lock before we went on vacation.

That makes it a very good example of the Icelandic pattern:

• láta einhvern gera eitthvað
• áður en
  • clause
• fara í frí