Ég kaupi þessa bók í búð.

I buy this book at a store.

Breakdown of Ég kaupi þessa bók í búð.

• ég: I
• kaupa: to buy
• búðin: the store
• bókin: the book
• í: at
• þessi: this

Questions & Answers about Ég kaupi þessa bók í búð.

Why is kaupi spelled with -i at the end? What form is it?

Kaupi is the 1st person singular present tense of the verb að kaupa (to buy).
Present tense endings for að kaupa are:

• ég kaupi (I buy)
• þú kaupir (you buy, singular)
• hann/hún/það kaupir (he/she/it buys)
• við kaupum (we buy)
• þið kaupið (you buy, plural)
• þeir/þær/þau kaupa (they buy)

So the -i ending marks I in the present tense for many verbs.

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Does this sentence mean present or future? Could it also mean I’m going to buy?

Grammatically it’s present tense: Ég kaupi ... = I buy / I am buying.
In Icelandic, the present tense can also be used for near-future plans in the right context (similar to English I’m buying it tomorrow), e.g. Ég kaupi hana á morgun (with á morgun making the future meaning clear).
If you want an explicit future intention, you might also hear Ég ætla að kaupa ... (I intend/am going to buy ...).

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Why is it þessa bók and not þessi bók?

Because bók is the direct object of kaupi (I buy something), it takes the accusative case.
The demonstrative þessi (this) changes form to match case + gender + number:

For feminine singular (bók is feminine), the common forms are:

• Nominative: þessi (subject form)
• Accusative: þessa (object form) ← used here
• Dative: þessari
• Genitive: þessarar

So þessa bók = accusative feminine singular.

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How do I know bók is feminine, and does that affect anything else?

Bók is feminine, and that affects:

• The form of þessi → þessa
• Adjectives and articles you might add
• Pronouns referring back to it (typically hún for a feminine noun)

A quick way to check gender is a dictionary entry (often marked f. for feminine). Over time you’ll also learn common gender patterns, but dictionary confirmation is the reliable method.

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Why doesn’t Icelandic use a/the here? Shouldn’t it be a shop or the shop?

Icelandic has no separate word for a/an (indefinite article). A bare noun like búð can mean a shop depending on context.
For the shop, Icelandic usually adds a definite ending to the noun:

• búð = a shop
• búðin = the shop And in this sentence, with í, it becomes:
• í búð = in a shop / at a shop
• í búðinni = in the shop

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Why is it í búð and not í búðina? What case does í take?

The preposition í can take dative or accusative depending on meaning:

• í + dative = location (being in/at a place)
• í + accusative = motion into (going into a place)

Here, Ég kaupi ... í búð describes where the buying happens (location), so it uses dative: búð → búð (dative singular is the same form as nominative for some nouns).
If you meant motion into the shop, you’d use accusative:

• Ég fer í búðina (I go into the shop)

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Why is the word order Ég kaupi þessa bók í búð? Is the placement of í búð flexible?

The basic Icelandic word order here is: Subject + verb + object + place phrase
So Ég (subject) + kaupi (verb) + þessa bók (object) + í búð (place).

The place phrase can sometimes move for emphasis, but Icelandic has stricter rules when something other than the subject comes first (then the verb must be second). For example:

• Í búð kaupi ég þessa bók. (In a shop, I buy this book.)
  Here the verb kaupi stays second.

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What’s the difference between þessi, þetta, and þessi? (They look repeated!)

They are gender/number forms of the same demonstrative þessi (this):

• Masculine singular nominative: þessi
• Feminine singular nominative: þessi
• Neuter singular nominative: þetta

Then each of those declines by case. In your sentence you see þessa, which is the accusative feminine singular form (matching bók).

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How would I make it negative?

You add ekki (not), typically after the verb (or after the object, depending on emphasis):

Common:

• Ég kaupi ekki þessa bók í búð. (I do not buy this book in a shop.)

Emphasis on what you’re not buying:

• Ég kaupi þessa bók ekki í búð. (Less common; more contrastive, like: not in a shop.)

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How do I turn it into a yes/no question?

You usually invert verb and subject (verb first):

• Kaupi ég þessa bók í búð? (Do I buy this book in a shop?)

In real usage, you might more often ask about the other person:

• Kaupir þú þessa bók í búð? (Do you buy this book in a shop?)

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How would I say I’m buying this book at the shop (more like a specific shop we both know)?

Use the definite form búðin (the shop), which in dative becomes búðinni:

• Ég kaupi þessa bók í búðinni. (I buy/am buying this book in the shop.)

That signals a specific shop (known from context).

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Can I replace Ég with something else, and does the verb change?

Yes, and the verb must agree with the new subject:

Examples:

• Þú kaupir þessa bók í búð. (You buy...)
• Við kaupum þessa bók í búð. (We buy...)
• Þeir kaupa þessa bók í búð. (They buy...)

Icelandic verb endings are important, so changing the subject usually changes the verb form too.