Compro el catálogo en la tienda.

In Spanish, the subject pronoun is often omitted because the verb ending already tells you who the subject is.

• compro = I buy
• The -o ending shows it is first person singular: yo

So Compro el catálogo en la tienda is a complete sentence by itself.
You could say Yo compro el catálogo en la tienda, but yo is usually only added for emphasis, contrast, or clarity.

Compro is in the present indicative tense.

It comes from the verb comprar = to buy.

Present tense forms:

• yo compro = I buy
• tú compras = you buy
• él/ella compra = he/she buys
• nosotros compramos = we buy
• ustedes compran = you all buy
• ellos/ellas compran = they buy

So here, compro means I buy.

It can often mean either one, depending on context.

In Spanish, the simple present can express:

• a habitual action: I buy
• a current action: I am buying

So Compro el catálogo en la tienda could mean:

• I buy the catalog at the store
• I’m buying the catalog at the store

If you want to strongly emphasize that the action is happening right now, Spanish often uses:

• Estoy comprando el catálogo en la tienda = I am buying the catalog at the store

But the simple present is still very common.

Spanish uses articles much more often than English.

So where English might sometimes say:

• I buy catalogs
• I buy a catalog
• I buy the catalog

Spanish usually wants an article:

• compro el catálogo = I buy the catalog
• compro un catálogo = I buy a catalog

Here, el catálogo means a specific catalog, not just any catalog in general.

Because el is the definite article and un is the indefinite article.

• el catálogo = the catalog → a specific one
• un catálogo = a catalog → any one, not specifically identified

So this sentence uses el because it refers to a particular catalog.

Because en means in or at, and here it shows the location where the buying happens.

• en la tienda = in/at the store

Spanish uses a mainly to show movement toward a place:

• Voy a la tienda = I’m going to the store

But in your sentence, the action is not about going somewhere. It is about buying something at that location, so en is the natural choice:

• Compro el catálogo en la tienda = I buy the catalog at the store

Because tienda is a feminine noun in Spanish, so it takes the feminine article la.

• la tienda = the store

Many nouns ending in -a are feminine, and tienda is one of them.
That is why you say:

• la tienda not
• el tienda

You also see agreement with adjectives:

• la tienda grande = the big store

The accent mark tells you where the stress goes in pronunciation.

catálogo is stressed on the second syllable:

• ca-TÁ-lo-go

Without the accent mark, Spanish spelling rules would suggest a different stress pattern, so the accent is necessary.

It is also important in writing because changing or removing accent marks can create a misspelling.

Yes. Spanish word order is more flexible than English word order.

The neutral order here is:

• Compro el catálogo en la tienda.

But you could also say:

• En la tienda compro el catálogo.

That puts more emphasis on where the action happens.

Both are grammatical, but the original sentence sounds more neutral and straightforward.

Yes. Catálogo is a cognate of English catalog or catalogue.

That makes it easier to recognize and remember.
Just be aware of:

• the article: el catálogo
• the accent mark
• the pronunciation, especially the stressed syllable: ca-TÁ-lo-go

Yes. The Spanish present tense can describe a repeated or habitual action.

So depending on context, Compro el catálogo en la tienda could mean:

• I buy the catalog at the store as a general habit, or
• I’m buying the catalog at the store right now

Context usually tells you which meaning is intended.

The infinitive is comprar, which means to buy.

So:

• comprar = to buy
• compro = I buy

This is a regular -ar verb, so it follows the normal present-tense pattern:

• comprar
• yo compro
• tú compras
• él/ella compra, etc.

That makes it a good beginner verb to learn.