Learn Hardcore Icelandic: Ég kaupi póstkort í búð.

using the bare noun:
- póstkort = a postcard / postcards (depending on context)
- búð = a shop
or, when you really mean “one”, by using einn/ein/eitt (= one):
- Ég kaupi eitt póstkort. = I buy one postcard.

So Ég kaupi póstkort í búð. can naturally mean I buy a postcard in a shop or I buy postcards in a shop.

How can póstkort mean both “postcard” and “postcards”?

Póstkort is a neuter noun whose singular and plural nominative/accusative forms are identical:

Singular: póstkort = postcard
Plural: póstkort = postcards

To show singular vs plural more clearly, Icelandic relies on context and on other words:

póstkortið = the postcard (singular definite)
póstkortin = the postcards (plural definite)
eitt póstkort = one postcard
nokkur póstkort = some postcards
mörg póstkort = many postcards

In your sentence, context decides whether you mean one postcard or postcards in general.

What grammatical case is póstkort in, and why?

In Ég kaupi póstkort í búð., póstkort is the direct object of the verb kaupi, so it is in the accusative case.

For neuter nouns like póstkort, the nominative and accusative look the same, so you don’t see the case on the form itself. You know it’s accusative because:

it is what is being bought (the thing acted on),
and most transitive verbs, including kaupa, take their object in the accusative.

What case is búð in after í, and why is it also just búð?

The preposition í (in/into) can take accusative or dative, depending on meaning:

í + accusative = motion into something
- Ég fer í búðina. = I go into the shop.
í + dative = location inside something
- Ég er í búð. = I am in a shop.

In Ég kaupi póstkort í búð., the meaning is in a shop (location), so í takes the dative.

The noun búð (feminine) happens to have the same form in nominative, accusative and dative singular:

nom: búð
acc: búð
dat: búð
gen: búðar

So it looks the same, but grammatically it’s dative here because of the preposition í plus the meaning “in” (not “into”).

Could I say “I am buying postcards in a shop” with Ég kaupi póstkort í búð, or is that only habitual?

Ég kaupi póstkort í búð. is present simple, and in Icelandic it most naturally suggests a habit or general fact:

I (usually) buy postcards in a shop.

To focus on something happening right now, Icelandic often uses “vera + að + infinitive”:

Ég er að kaupa póstkort í búð.
= I am buying postcards in a shop (right now).

In casual conversation, Ég kaupi… can sometimes be used for a current action if the context makes it obvious, but Ég er að kaupa… is the clearest “I’m in the middle of buying” form.

How flexible is the word order? Could I say Ég kaupi í búð póstkort?

Basic Icelandic word order in simple statements is:

Subject – Verb – Object – (other information)

So Ég kaupi póstkort í búð. is the most neutral:

Ég (subject)
kaupi (finite verb)
póstkort (object)
í búð (place phrase)

You can say Ég kaupi í búð póstkort, but it sounds a bit marked or less natural in everyday speech. Word order is somewhat flexible for emphasis and style, but as a learner you’re safest with:

Subject – Verb – Object – Place – Time

Here: Ég kaupi póstkort í búð.

What are the genders of póstkort and búð, and why does that matter?

póstkort is neuter.
búð is feminine.

Gender matters because it affects:

Definite endings
- póstkortið = the postcard (neuter sg. def.)
- búðin = the shop (feminine sg. nom./acc. def.)
- í búðinni = in the shop (feminine sg. dat. def.)
Adjective endings
- fallegt póstkort = a beautiful postcard (neuter)
- falleg búð = a beautiful shop (feminine)
Pronouns and agreement later on.

So learning the gender with each noun from the start is important:
(n.) póstkort, (f.) búð.

What does póstkort literally consist of? Is it a compound word?

Yes, póstkort is a compound noun:

póstur = post, mail
kort = card

Combined: póstkort = postcard.

In Icelandic, compounds are extremely common, and usually:

written as one word
stressed mainly on the first element: PÓSTkort

How do I pronounce Ég kaupi póstkort í búð?

Approximate pronunciation (not strict IPA), with main stress shown:

Ég ≈ yegh (like ye
- a soft voiced gh sound)
kaupi ≈ KOY-pi
- au is like English “oy” in boy, but shorter and tenser.
póstkort ≈ POWST-kort
- ó is like English “owe” in row, but shorter.
- st as in English, kort roughly kort.
í ≈ ee (long i as in see).
búð ≈ booth but shorter, with a soft ð at the end (like the th in this, but often very weak).

Word stress is almost always on the first syllable of each word:
ÉG kaupi PÓSTkort í BÚÐ.

How would the sentence change if I wanted to say “the postcard” or “in the shop”?

Using the definite form is done by adding endings to the noun:

the postcard = póstkortið
the shop (subject or direct object) = búðin
in the shop (dative) = í búðinni

Examples:

Ég kaupi póstkortið í búðinni.
= I buy the postcard in the shop.
Ég kaupi póstkort í búðinni.
= I buy postcards in the (specific) shop.

Compare with your original:

Ég kaupi póstkort í búð.
= I buy a postcard / postcards in a shop (non-specific).

Can I leave out Ég like in Spanish or Italian, and just say Kaupi póstkort í búð?

Normally, no. Icelandic is not a “pro-drop” language:

The subject pronoun (ég, þú, hann…) is usually expressed.
Kaupi póstkort í búð without Ég sounds more like a fragment, an instruction, or very telegraphic style.

In everyday speech and writing, you should say:

Ég kaupi póstkort í búð.

How would I say this sentence in the past or future?

The verb kaupa is regular in the past:

Ég keypti póstkort í búð.
= I bought postcards in a shop.

For a simple future meaning, Icelandic usually uses present tense with a future time word, or a modal like ætla (to intend), munu (will).

Common options:

Ég mun kaupa póstkort í búð.
= I will buy postcards in a shop.
Ég ætla að kaupa póstkort í búð.
= I’m going to buy postcards in a shop.

Often just:

Á morgun kaupi ég póstkort í búð.
= Tomorrow I buy / will buy postcards in a shop. (future meaning from á morgun).