Ég ætlaði bara að kíkja í bókabúðina, en endaði á að kaupa tvær orðabækur.

It is the past tense of ætla, a very common verb meaning intend, plan, or be going to.

So:

• ég ætla = I intend / I’m going to
• ég ætlaði = I intended / I was going to

In this sentence, Ég ætlaði bara að... is very naturally understood as I was just going to... or I only meant to...

Here að is the infinitive marker, like English to.

So:

• ætla að kíkja = intend to look / intend to pop in

After ætla, you normally use að + infinitive:

• Ég ætla að fara = I’m going to go
• Ég ætlaði að kaupa = I intended to buy

This að is not the same as the preposition að meaning at/to.

Bara usually means just, only, or simply.

In this sentence it softens the statement and gives the idea that the speaker had a small, harmless plan:

• Ég ætlaði bara að kíkja... = I was just going to pop in...

It suggests: that was all I meant to do.
That makes the second half funnier or more relatable: the speaker only meant to look, but ended up buying two dictionaries.

Kíkja literally has the sense of peek, look briefly, or take a quick look.
With í, it often means something like:

• have a quick look in
• pop into
• drop by
• check out briefly

So kíkja í bókabúðina is not as neutral as simply go to the bookstore. It suggests a short, casual visit.

That is why it fits well with bara: the speaker only meant to pop into the bookstore for a moment.

This is about case after í.

The preposition í can take:

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

Here we have bókabúðina, which is accusative singular definite. The idea is into the bookstore / pop into the bookstore.

Compare:

• í bókabúðina = into the bookstore
• í bókabúðinni = in the bookstore

So the sentence is presenting the bookstore as the destination of the quick visit.

Because Icelandic uses compounds very freely, much more than English.

Bókabúðina can be broken down like this:

• bóka- = book/books-related first element
• búð = shop/store
• -ina = the definite ending here

So bókabúðina means the bookstore.

English usually writes this as two words or uses a phrase, but Icelandic often packs the whole idea into one compound noun.

The same thing happens with orðabók:

• orða- = word(s)
• bók = book

So orðabók literally looks like word-book, i.e. dictionary.

No. In this sentence, á að is part of the fixed expression enda á að + infinitive, which means:

• end up doing
• finish by doing

So:

• endaði á að kaupa = ended up buying

This is different from the separate construction að eiga að or vera að fara að, and also different from á að meaning is supposed to in other contexts.

So here you should learn enda á að as one useful pattern:

• Hann endaði á að missa af rútunni. = He ended up missing the bus.
• Við enduðum á að vera heima. = We ended up staying home.

Because Icelandic often omits a repeated subject in coordinated clauses when it is already obvious.

So:

• Ég ætlaði bara að kíkja í bókabúðina, en endaði á að kaupa...

means the same as:

• Ég ætlaði bara að kíkja í bókabúðina, en ég endaði á að kaupa...

The second ég is understood from the first clause.
Leaving it out sounds natural and avoids repetition.

If you do repeat ég, it can sound a bit more emphatic.

Because orðabók is a feminine noun, and the number 2 changes for gender in Icelandic:

• tveir = masculine
• tvær = feminine
• tvö = neuter

Since orðabók is feminine, you need tvær:

• tvær orðabækur = two dictionaries

This is one of the first places where Icelandic gender really matters in everyday sentences.

Because after 2, Icelandic uses the plural noun.

So:

• ein orðabók = one dictionary
• tvær orðabækur = two dictionaries

Also, the whole phrase is the direct object of kaupa (buy), so it is in the appropriate object case. In this particular phrase, the forms you see are the normal ones you would expect here.

The important thing for a learner is:

• 2, 3, 4 are followed by plural nouns
• the numeral agrees with the noun’s gender

Because bók has an irregular plural:

• bók = book
• bækur = books

That vowel change is a normal historical pattern in Icelandic.
Since orðabók is built on bók, it follows the same pattern:

• orðabók = dictionary
• orðabækur = dictionaries

So once you know:

• bók → bækur

then:

• orðabók → orðabækur

will feel much more logical.

Yes, very natural.

A few things are happening:

• Ég ætlaði...: the finite verb ætlaði comes early, as expected in a normal main clause
• bara sits before að kíkja, which is a natural place for just
• after en (but), the second clause continues smoothly with the understood same subject
• endaði á að kaupa... is a very natural way to express ended up buying...

So the sentence sounds idiomatic and conversational, not overly formal or textbook-like.