Ég ætlaði að kaupa bara nítján blöðrur, en endaði á að taka tuttugu.

It means I intended to or I was going to.

• ég = I
• ætlaði = past tense of ætla, meaning intend / plan
• að kaupa = to buy

So Ég ætlaði að kaupa... is literally I intended to buy...

A useful thing to notice: ætla is very commonly followed by að + infinitive, much like English to + verb.

Because there are two separate infinitive constructions:

1. ætlaði að kaupa = intended to buy
2. endaði á að taka = ended up taking

In both places, að introduces an infinitive verb.

But the second one is slightly different: in endaði á að taka, the á belongs to the fixed expression enda á að + verb, which means to end up doing something.

So the two aðs are not accidental repetition; each one belongs to its own verb phrase.

As a whole, endaði á að means ended up.

So:

• endaði á að taka tuttugu = ended up taking twenty

You should learn this as a chunk:

enda á að + infinitive = end up doing

If you translate it word-for-word, it may seem strange, but idiomatically it is very natural Icelandic.

Because the two verbs highlight slightly different things:

• kaupa = buy
• taka = take / pick / choose / take along

So the sentence is saying:

• the speaker's plan was to buy nineteen balloons,
• but in the actual situation, they ended up taking twenty.

In English, we might repeat buy, but Icelandic often uses taka naturally when talking about what someone finally selected or took. Here it implies the balloons were the ones actually chosen, and buying is understood from the context.

Because bara is modifying the number.

Here it means only / just, so:

• bara nítján blöðrur = only nineteen balloons

Its position shows what is being limited. Putting bara right before nítján makes it clear that the restriction is on the quantity.

If you move bara, the basic meaning may stay similar, but the emphasis can shift slightly.

Because the sentence is talking about more than one balloon, so the noun must be plural.

• singular: blaðra = balloon
• plural: blöðrur = balloons

This noun also shows a stem vowel change:

• blaðra → blöðrur

That kind of vowel change is common in Icelandic noun patterns.

In this sentence, blöðrur is the direct object of kaupa, so it is in the plural object form. For this particular noun, the nominative plural and accusative plural happen to look the same: blöðrur.

Because the noun is understood from the earlier part of the sentence.

• taka tuttugu here means take twenty [balloons]

Icelandic, like English, often leaves out a repeated noun when it is already obvious from context.

So the full idea is:

• endaði á að taka tuttugu blöðrur

But saying just tuttugu is completely natural because blöðrur has already been mentioned.

Here they function simply as quantity words:

• nítján blöðrur = nineteen balloons
• tuttugu = twenty (with blöðrur understood)

A useful learner point is that numbers like nítján and tuttugu are much simpler here than low numbers such as one, two, three, and four, which often change more for gender and case.

So in this sentence, you can mainly focus on the pattern:

number + plural noun

Yes:

• Ég = I
• ætlaði að kaupa = intended to buy / was going to buy
• bara nítján blöðrur = only nineteen balloons
• en = but
• endaði á að taka tuttugu = ended up taking twenty

So the structure is:

I intended to buy only nineteen balloons, but ended up taking twenty.

That is a very natural Icelandic way to express a plan that changed in the end.