Ich kaufe den Katalog im Geschäft.

I buy the catalogue in the store.

Breakdown of Ich kaufe den Katalog im Geschäft.

• in: in
• dem: the; (masculine or neuter, dative)
• ich: I
• kaufen: to buy
• das Geschäft: the store
• der Katalog: the catalogue

Questions & Answers about Ich kaufe den Katalog im Geschäft.

Why is it den Katalog instead of der Katalog, and can I say einen Katalog or drop the article?

In German, Katalog is a masculine noun (nominative der Katalog). When it’s the direct object of kaufen, it takes the accusative case, so the definite article changes to den. German normally requires an article or other determiner with singular, countable nouns, so you cannot drop it. If you want to say “a catalog,” use the masculine accusative indefinite article einen:
Ich kaufe einen Katalog im Geschäft.

What case is used in im Geschäft, and why is im used instead of in dem?

im is simply the contraction of in dem (in + dative). When in expresses a static location (where something happens), it governs the dative case. Hence dem Geschäft becomes im Geschäft.

Why not use ins Geschäft in this sentence?

ins is the contraction of in das, which puts Geschäft in the accusative case, implying movement into the store. Here, buying takes place inside the store (a location), not emphasizing going into it. For static location you need im (in + dative), not ins (in + accusative).

Can the adverbial phrase im Geschäft be placed somewhere else in the sentence?

Yes. German word order is flexible with adverbials. You can say:
Im Geschäft kaufe ich den Katalog.
Or:
Ich kaufe im Geschäft den Katalog.
In all cases the finite verb must remain in the second position in a main clause.

Why is kaufe in the second position, and not at the end of the sentence?

German main clauses follow the verb‑second (V2) rule: the finite verb always appears as the second element, no matter which part you put first. Here Ich is the first element, so kaufe must come second.

Why is Geschäft capitalized, but ich is not (except when it’s first)?

In German, all nouns are capitalized wherever they appear. Pronouns (like ich) are only capitalized if they start a sentence; otherwise they remain lowercase.

When would I use bei instead of im to say “at the store” in German?

Use im (in + dative) to emphasize being or doing something inside a physical place:
Ich kaufe den Katalog im Geschäft.
Use bei + dative to indicate proximity or association with a person or business (not necessarily inside):
Ich bestelle das Buch bei Amazon.

What’s the difference between Geschäft, Laden, and Kaufhaus?

Geschäft is a general term for any shop or business.
Laden usually refers to a small, specialized shop.
Kaufhaus means department store, a large establishment with many departments under one roof.