Það er stutt síðan hún keypti ný sundföt fyrir ferðina.

Það er stutt síðan ... is a very common Icelandic way to say It has not been long since ... or It’s only a short time since ...

So:

• Það er = it is / there is
• stutt = short
• síðan = since

Taken together, Það er stutt síðan hún keypti ... means something like:

• It hasn’t been long since she bought ...
• She bought ... recently

This is a fixed, natural Icelandic time expression, and it does not translate word-for-word very neatly into natural English.

Because stutt is the neuter singular form of the adjective stuttur (short).

In Það er stutt síðan ..., the adjective is used in an impersonal construction with það, and Icelandic normally uses the neuter singular in this kind of pattern.

So:

• stuttur = masculine
• stutt = neuter
• stutt is the form needed here

You can think of it as similar to how Icelandic often uses neuter singular in general statements or impersonal expressions.

Because the sentence is talking about the present situation: the amount of time that has passed up to now.

So:

• Það er stutt síðan ... = It is only a short time since ...

The event itself is in the past, but the expression measures the time between that past event and now, so the main verb is present tense:

• er = is

This is very similar to English:

• It is a long time since I saw him
• It has been a short time since she bought ...

Because keypti refers to the action that happened earlier: she bought.

In this pattern, Icelandic often uses:

• present tense in the main clause: Það er stutt síðan ...
• past tense in the clause after síðan: hún keypti ...

That is because the buying happened before now.

So the structure is:

• Now: it is only a short time
• Earlier event: she bought new swimwear

Keypti is the past tense of the verb að kaupa (to buy).

A few useful forms are:

• að kaupa = to buy
• kaupir = buys / is buying
• keypti = bought
• keypt = bought (past participle)

So in this sentence:

• hún keypti = she bought

Because sundföt is a plural noun, and it usually means swimwear / swimsuit / bathing clothes as a set.

So:

• sundfat = a single bathing garment item (less common in everyday use for a whole swimsuit)
• sundföt = swimwear, bathing clothes, swimsuit(s)

Since sundföt is plural, the adjective must also be plural:

• ný = new in the neuter plural form

That is why we get:

• ný sundföt = new swimwear / a new swimsuit

Even if English often uses a singular word like swimsuit, Icelandic commonly uses this plural form.

Sundföt is a neuter plural noun.

The adjective nýr (new) changes to match the noun in gender, number, and case. Here it must agree with sundföt:

• singular neuter: nýtt
• plural neuter: ný

So:

• nýtt sundfat = a new bathing item
• ný sundföt = new swimwear

In this sentence, ný is the correct neuter plural form.

Ferðina is the accusative singular definite form of ferð (trip, journey).

The preposition fyrir can take different cases depending on meaning, but here it means for in the sense of intended for or for the purpose of, and that use takes the accusative.

So:

• ferð = trip
• ferðin = the trip
• ferðina = the trip (accusative)

Thus:

• fyrir ferðina = for the trip

Here fyrir means for, as in:

• intended for
• in preparation for
• for the sake of

So ný sundföt fyrir ferðina means she bought the swimwear for the trip.

It does not mean before the trip here. If you want to say before the trip, Icelandic would usually express that differently, for example with fyrir ferðina only in a time sense if the context clearly supports it, but in this sentence the natural meaning is for the trip.

This is the normal word order for this kind of statement.

Basic structure:

• Það er stutt síðan = fixed introductory time expression
• hún keypti ný sundföt fyrir ferðina = the clause describing the past event

So the sentence is built as:

1. impersonal opening
2. time relation
3. the event

You could move things around in some contexts for emphasis, but this order is the most neutral and natural.

Often, but not always. Síðan can have a few related uses, such as:

• since
• after that / then
• part of time expressions like Það er langt/stutt síðan ...

In this sentence, it is specifically part of the pattern meaning:

• it has been a short time since ...

So here it is best understood as since.

Yes. That is a very natural English rendering.

Even though the Icelandic literally uses a structure like It is short since she bought..., the real meaning is simply that the buying happened recently.

So good English translations include:

• It hasn’t been long since she bought new swimwear for the trip.
• She recently bought new swimwear for the trip.

The second one is less literal, but it captures the meaning very well.