Hún lét saumakonuna laga faldinn áður en hún fór í brúðkaupið.

Lét is the past tense of láta.

In this sentence, láta + person/object + infinitive is a very common way to express a causative idea:
to have/get/make someone do something.

So:

• Hún lét saumakonuna laga faldinn
  = She had the seamstress fix the hem

Depending on context, láta can sometimes be translated as let, make, or have. Here, English usually prefers had rather than let:

• She had the seamstress fix the hem
  more natural here than
• She let the seamstress fix the hem

Because it is the direct object of lét.

The base word is saumakona = seamstress.
Saumakonuna means the seamstress in the accusative singular.

So the structure is:

• Hún lét saumakonuna ...
  She had the seamstress ...

This is a very typical Icelandic pattern:

• subject in nominative: hún
• direct object in accusative: saumakonuna

After láta in this causative construction, Icelandic normally uses the bare infinitive.

So you get:

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

Here:

• lét saumakonuna laga faldinn

not:

• lét saumakonuna að laga faldinn

So laga stays in the infinitive, and að is not used.

Because faldinn is the direct object of the infinitive laga.

So the sentence contains two linked parts:

1. Hún lét saumakonuna ...
   = She had the seamstress ...

2. ... laga faldinn
   = ... fix the hem

The base noun is faldur = hem.
Faldinn means the hem in the accusative singular.

So:

• saumakonuna = the person being made/asked to do the action
• faldinn = the thing being fixed

Because Icelandic usually puts the definite article onto the end of the noun instead of using a separate word like English the.

So:

• saumakona = seamstress

• saumakonan / saumakonuna = the seamstress

• faldur = hem

• faldurinn / faldinn = the hem

• brúðkaup = wedding
• brúðkaupið = the wedding

The exact form changes depending on case, gender, and number.

Áður en means before.

It introduces a subordinate clause with a finite verb:

• áður en hún fór í brúðkaupið
  = before she went to the wedding

So the pattern is:

• main clause + áður en + full clause

Here:

• main clause: Hún lét saumakonuna laga faldinn
• subordinate clause: áður en hún fór í brúðkaupið

This is very close to English before she went...

Here í is used with the accusative because there is movement toward an event or destination.

• fór í brúðkaupið
  = went to the wedding

A very important Icelandic rule is that many prepositions, including í, can take:

• accusative for motion/direction
• dative for location/state

So compare the general idea:

• fara í ... = go into/to ...
• vera í ... = be in ...

In this sentence she is going to the wedding, so í takes the accusative form: brúðkaupið.

Yes, grammatically it can be.

• Hún lét saumakonuna laga faldinn áður en hún fór í brúðkaupið.

The second hún could in principle refer to:

• the main subject, she
• or the seamstress

In real usage, context usually makes it clear. Most readers will probably assume it refers to the main subject unless the wider context suggests otherwise.

So this is one of those sentences where Icelandic, like English, can be ambiguous:

• She had the seamstress fix the hem before she went to the wedding.

English has the same problem: who went to the wedding?

Sometimes yes, but here had is the best translation.

Compare:

• Hún lét saumakonuna laga faldinn.
  Usually: She had the seamstress fix the hem.

If you say She made the seamstress fix the hem, that sounds stronger, as if the seamstress resisted and was forced.

If you say She let the seamstress fix the hem, that suggests permission.

So láta covers a range that English splits into different verbs. In this sentence, the natural English choice is had.

Here are the key dictionary forms:

• hún = she
• láta = to let, make, have
• saumakona = seamstress
• laga = to fix, repair, mend, adjust
• faldur = hem
• áður en = before
• fara = to go
• brúðkaup = wedding

And the forms actually used in the sentence are:

• lét = past tense of láta
• saumakonuna = accusative singular definite of saumakona
• faldinn = accusative singular definite of faldur
• fór = past tense of fara
• brúðkaupið = definite form of brúðkaup here used after í

The sentence breaks down like this:

• Hún = subject
• lét saumakonuna laga faldinn = causative construction
  (had the seamstress fix the hem)
• áður en hún fór í brúðkaupið = time clause
  (before she went to the wedding)

So the full pattern is:

• Subject + láta in past tense + person + infinitive + object + áður en + clause

That makes it a very useful model sentence for learning how Icelandic expresses having someone do something.