Hún vill láta lyklasmiðinn athuga lásinn aftur, því hurðin læsist ekki alltaf.

This is a very common Icelandic pattern:

láta + person/object + infinitive

It often means have someone do something, make someone do something, or sometimes let someone do something, depending on context.

So here:

Hún vill láta lyklasmiðinn athuga lásinn = She wants to have the locksmith check the lock

In natural English, have the locksmith check is the best match here.

vill is the 3rd person singular present of the verb vilja = to want.

So:

• ég vil = I want
• þú vilt = you want
• hún vill = she wants

That is why the sentence starts with Hún vill = She wants.

The base noun is lyklasmiður = locksmith.

In the nominative singular definite (the subject form, the locksmith), you get:

• lyklasmiðurinn

But in this sentence, the locksmith is not the subject. He is the person being made/asked to do the action after láta, and that construction uses the accusative.

So the form becomes:

• lyklasmiðinn

This is a very useful thing to notice: many masculine nouns in -ur lose that -ur in oblique cases.

It is a compound noun:

• lykill = key
• smiður = smith / craftsman

So lyklasmiður literally means something like key-smith, i.e. locksmith.

Compound nouns are extremely common in Icelandic, so this is a good word to break apart and learn.

Because after láta, Icelandic uses a bare infinitive.

So you say:

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

not

• láta einhvern að gera eitthvað

That is why the sentence has:

• láta lyklasmiðinn athuga
• not láta lyklasmiðinn að athuga

This is similar to English make/let/have someone do rather than make someone to do.

Because it depends on láta.

After láta, the following verb stays in the infinitive:

• láta athuga = have someone check
• láta laga = have someone repair
• láta opna = have someone open

So athuga is not a finite verb here. It is part of the construction láta ... athuga.

lás means lock.

With the suffixed definite article, it becomes:

• lásinn = the lock

In this sentence, it is the direct object of athuga, so it is in the accusative.

For this noun, the definite nominative and accusative singular happen to look the same:

• nominative: lásinn
• accusative: lásinn

So even though the form does not change visibly, its role here is object: check the lock.

Here aftur means again.

So:

• athuga lásinn aftur = check the lock again

It refers to repeating the checking. In other contexts, aftur can also mean back, but here again is clearly the right meaning.

Its position is fairly natural here after the object, though adverb placement in Icelandic can be somewhat flexible.

Here því means because or since.

So:

• því hurðin læsist ekki alltaf = because the door doesn’t always lock

Yes, the form is the same as the dative of það, but in this sentence it is functioning as a conjunction, not as a pronoun.

Learners also often see:

• því að = because
• af því að = because

All of those can introduce an explanation.

The noun is hurð = door.

With the suffixed definite article:

• hurðin = the door

Here it is the subject of the second clause:

• hurðin læsist ekki alltaf = the door doesn’t always lock

So hurðin is in the nominative singular definite.

læsist is from læsast, an -st form related to læsa = to lock.

In this sentence, læsist means something like:

• locks
• can be locked properly
• catches/locks as it should

So:

hurðin læsist ekki alltaf means the door doesn’t always lock

This does not mean exactly the same as is not always locked.

Compare:

• Hurðin læsist ekki alltaf = the door does not always lock properly
• Hurðin er ekki alltaf læst = the door is not always in a locked state

That distinction is very important.

Because Icelandic expresses not always as:

• ekki alltaf

Here:

• læsist ekki alltaf = doesn’t always lock

The negation ekki comes before alltaf in this normal phrasing.

So the idea is not never; it is sometimes yes, sometimes no.

Because after því meaning because, the normal neutral order is subject + verb:

• hurðin læsist ekki alltaf

So the sentence continues naturally as:

• því hurðin læsist ekki alltaf

Putting the verb first there would sound marked, and in many contexts more like a question pattern than a neutral explanatory clause.

So for a learner, the safe takeaway is:

• after því = because, use normal clause order
• því hurðin læsist ekki alltaf = natural
• því læsist hurðin ekki alltaf = not the neutral choice here