Kaupi is the present tense, 1st person singular form of the verb kaupa (to buy).
So from kaupi, you can tell the subject must be ég (I), even if it weren’t written.
Icelandic has no separate word for “a/an”. Indefiniteness is usually shown by:
So Ég kaupi póstkort í búð. can naturally mean I buy a postcard in a shop or I buy postcards in a shop.
Póstkort is a neuter noun whose singular and plural nominative/accusative forms are identical:
To show singular vs plural more clearly, Icelandic relies on context and on other words:
In your sentence, context decides whether you mean one postcard or postcards in general.
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:
The preposition í (in/into) can take accusative or dative, depending on meaning:
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:
So it looks the same, but grammatically it’s dative here because of the preposition í plus the meaning “in” (not “into”).
Ég kaupi póstkort í búð. is present simple, and in Icelandic it most naturally suggests a habit or general fact:
To focus on something happening right now, Icelandic often uses “vera + að + infinitive”:
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.
Basic Icelandic word order in simple statements is:
So Ég kaupi póstkort í búð. is the most neutral:
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:
Here: Ég kaupi póstkort í búð.
Gender matters because it affects:
Definite endings
Adjective endings
Pronouns and agreement later on.
So learning the gender with each noun from the start is important:
(n.) póstkort, (f.) búð.
Yes, póstkort is a compound noun:
Combined: póstkort = postcard.
In Icelandic, compounds are extremely common, and usually:
Approximate pronunciation (not strict IPA), with main stress shown:
Word stress is almost always on the first syllable of each word:
ÉG kaupi PÓSTkort í BÚÐ.
Using the definite form is done by adding endings to the noun:
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:
Normally, no. Icelandic is not a “pro-drop” language:
In everyday speech and writing, you should say:
The verb kaupa is regular in the past:
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: