Kupila sam dvije razglednice, ali ti ih još nisam poslala jer nisam imala omotnicu.

Croatian often leaves out subject pronouns when they are already clear from the verb form. Here sam shows first person singular, so Kupila sam... already means I bought....

You could add ja for emphasis or contrast: Ja sam kupila dvije razglednice... But without emphasis, leaving it out is more natural.

These are forms of the Croatian perfect, the most common past tense in everyday Croatian.

It is formed with:

• a present-tense form of biti (sam, nisam)
• plus a past participle (kupila, poslala, imala)

So:

• kupila sam = I bought
• nisam poslala = I didn’t send / I haven’t sent
• nisam imala = I didn’t have

English translation depends on context.

Because the speaker is female. In Croatian, the past participle agrees with the gender and number of the subject.

So a woman says:

• kupila sam
• poslala sam
• imala sam

A man would say:

• kupio sam
• poslao sam
• imao sam

Because razglednica is a feminine noun.

With two, Croatian uses:

• dva for masculine and neuter nouns
• dvije for feminine nouns

So:

• dva grada = two cities
• dva sela = two villages
• dvije razglednice = two postcards

After dva/dvije, tri, četiri, Croatian uses a special counting form for the noun. With many feminine nouns, that form looks like razglednice.

Compare:

• razglednica = postcard
• razglednicu = postcard (accusative singular, for one postcard)
• dvije razglednice = two postcards

So if you say I bought one postcard, you get Kupila sam razglednicu. But with two, it becomes Kupila sam dvije razglednice.

They are short pronoun forms:

• ti = to you (dative singular)
• ih = them (accusative plural)

So ti ih poslala literally means something like sent them to you.

In full stressed forms, you would have:

• tebi = to you
• njih = them

But Croatian usually prefers the short unstressed forms in sentences like this.

Croatian short pronouns follow a fixed clitic order. When both a dative and an accusative clitic appear, the dative normally comes first.

So:

• ti ih = to you them
• mu ga = to him it
• joj ih = to her them

That is why ti ih is the normal order here.

Because they are clitics: short unstressed words that usually appear near the beginning of the clause.

That is why Croatian says: ali ti ih još nisam poslala

This is very natural Croatian word order. English does not work this way, so it often feels unusual to learners at first.

Here još with a negative verb gives the idea of still not or not yet.

So još nisam poslala can be translated as:

• I still haven’t sent
• I haven’t sent yet

Both are good English translations in this context. The idea is that up to now, the sending has not happened.

Because Croatian has special negative forms of biti:

• nisam
• nisi
• nije
• nismo
• niste
• nisu

So you say:

• nisam poslala
• not ne sam poslala

This is something you simply have to learn as a fixed pattern.

Because poslati is a perfective verb, and it refers to a completed sending event. Here the speaker means that the postcards have not been sent as completed items.

slati is the imperfective partner. nisam slala would suggest a different meaning, more like:

• I wasn’t sending
• I didn’t use to send
• I was not engaged in sending

So nisam poslala is the right choice for I haven’t sent them.

Because imati is an imperfective verb and describes a state: having or not having something.

The speaker is explaining the situation: because I didn’t have an envelope

This is not a single completed action like buy or send. It is a background state, so imala is exactly what you would expect.

Because omotnica is the direct object of imati, so it appears here in the accusative singular.

The forms are:

• omotnica = nominative
• omotnicu = accusative

So:

• Imam omotnicu = I have an envelope
• Nisam imala omotnicu = I didn’t have an envelope

Yes, Croatian word order is quite flexible, but not completely free. You can move parts of the sentence to change emphasis.

For example, you might also hear: Kupila sam dvije razglednice, ali još ti ih nisam poslala...

That puts more focus on još. But the short pronouns still follow normal clitic rules, so not every order is possible.