Sedamnaestog srpnja šaljem razglednicu baki i kupujem dvije marke u pošti.

Because Croatian dates are usually expressed with:

• an ordinal number for the day
• the month in the genitive

So sedamnaestog srpnja means on the seventeenth of July.

More literally:

• sedamnaestog = of the seventeenth
• srpnja = of July

The form sedamnaestog is genitive because it agrees with an implied word like dana (day), which is often left out.

Because after the day of the month, the month name is normally in the genitive singular.

So:

• srpanj = July
• srpnja = of July

Other examples:

• prvog svibnja = on the first of May
• petog rujna = on the fifth of September

Baki is the dative singular of baka (grandmother / grandma).

The dative is used for the recipient of something. Since the postcard is being sent to grandma, Croatian uses the dative:

• šaljem razglednicu baki = I am sending a postcard to grandma

Compare:

• baka = grandma
• baki = to grandma

Because razglednicu is the accusative singular form.

The verb šaljem (I send / I am sending) takes a direct object, and direct objects are often in the accusative.

So:

• razglednica = postcard
• razglednicu = postcard, as the thing being sent

Because marka is a feminine noun, and the number two must match that gender.

So:

• dva is used with masculine and neuter nouns
• dvije is used with feminine nouns

Examples:

• dva grada = two cities
• dva pisma = two letters
• dvije marke = two stamps

Also, after 2, 3, 4, Croatian uses a special plural-type form:

• jedna marka
• dvije marke
• pet marki

Here marke means postage stamps.

The singular is:

• marka = stamp

In other contexts, marka can also mean brand, so context matters.

In a sentence with razglednicu and u pošti, the meaning is clearly stamps.

Because u can take different cases depending on the meaning:

• u + accusative = movement into somewhere
• u + locative = being in / at somewhere

Here the meaning is at the post office, so Croatian uses the locative:

• u pošti = in/at the post office

Compare:

• idem u poštu = I am going to the post office
• kupujem marke u pošti = I buy stamps at the post office

Because Croatian often drops subject pronouns when they are clear from the verb ending.

Both verbs show 1st person singular:

• šaljem = I send / I am sending
• kupujem = I buy / I am buying

So ja is not necessary. You could say Ja šaljem..., but it usually adds emphasis.

This is very common in Croatian.

Grammatically, it is present tense:

• šaljem = I send / I am sending
• kupujem = I buy / I am buying

But with a date like sedamnaestog srpnja, it can refer to a planned future action, depending on context.

So the sentence can sound like:

• On July 17th, I’m sending a postcard to grandma and buying two stamps at the post office.

Croatian often uses the present for scheduled or planned actions.

Yes. Croatian word order is more flexible than English because the case endings show the grammatical roles.

This sentence is natural:

• Sedamnaestog srpnja šaljem razglednicu baki i kupujem dvije marke u pošti.

But other orders are possible, for example:

• Šaljem baki razglednicu sedamnaestog srpnja i kupujem dvije marke u pošti.

Different word orders can change emphasis, but not the basic meaning.

Putting Sedamnaestog srpnja first emphasizes the time.

Because Croatian does not have articles.

So Croatian simply says:

• šaljem razglednicu = I am sending a/the postcard
• u pošti = in/at the post office

Whether English needs a or the depends on context, but Croatian does not mark that distinction with articles.

In Croatian, months are normally not capitalized unless they begin a sentence.

So:

• srpanj
• kolovoz
• rujan

This is different from English, where month names always take a capital letter.

A few useful points:

• š sounds like sh in shoe
• lj is a single sound, similar to the lli in million for many learners
• nj is like the ny sound in canyon
• č and ć are both sounds English speakers often hear as versions of ch, though Croatian distinguishes them

Approximate pronunciations:

• šaljem ≈ SHAH-lyem
• srpnja ≈ SRP-nya
• pošti ≈ POSH-tee

These are only rough English-based approximations, but they can help you get started.

Yes, both are imperfective:

• slati / slati → šaljem = to send
• kupovati / kupiti? In this form kupujem comes from kupovati/kupiti-related usage, and in everyday learning it is usually understood as the normal present I buy / I am buying

For this sentence, the important point is that the present forms describe the action as an event in progress, habitual, or planned, depending on context.

A beginner does not need to worry too much here; the key thing is:

• šaljem = I send / I’m sending
• kupujem = I buy / I’m buying

The exact aspect system becomes more important later when you compare pairs like:

• pisati / napisati
• kupovati / kupiti

Yes. Both are grammatical.

• šaljem razglednicu baki
• šaljem baki razglednicu

Because Croatian uses case endings, both orders are clear. The difference is mainly style or emphasis.

• razglednicu earlier may highlight what you are sending
• baki earlier may highlight to whom

Both are natural.