Ana i Sara često šalju jedna drugoj poruke kad žele podršku.

Because Ana i Sara is a plural subject, so the verb must be in the 3rd person plural.

• šalju = they send
• šalje = he/she sends

The same thing happens later with žele:

• žele = they want
• želi = he/she wants

Jedna drugoj means to each other or more literally one to the other.

In this sentence, it shows that Ana and Sara send messages to one another, not just to someone else.

A very literal breakdown is:

• jedna = one (feminine)
• drugoj = to the other (feminine dative)

So the phrase expresses reciprocity: each other.

Because Ana and Sara are both female, so Croatian uses feminine forms.

Here are the matching forms:

• feminine: jedna drugoj
• masculine or mixed group: jedan drugome / jedan drugom

English each other does not change for gender, but Croatian does.

That is just how this Croatian reciprocal expression works. Croatian says the equivalent of one to the other, using singular forms, even when the meaning is plural overall.

So although Ana and Sara are two people, the expression is still:

• jedna drugoj = one to the other / each other

This is normal and idiomatic.

Drugoj is in the dative case.

That is because with slati / poslati (to send), the person who receives something is normally in the dative:

• šalju poruke Sari = they send messages to Sara
• šalju jedna drugoj poruke = they send each other messages

So:

• poruke = the thing being sent
• drugoj = the recipient

Here poruke is the direct object, so it is in the accusative plural.

The singular noun is:

• poruka = message

Plural:

• poruke = messages

In this sentence, poruke is what Ana and Sara are sending, so it is the object of šalju.

Podršku is the accusative singular of podrška (support).

It appears in the accusative because it is the direct object of žele:

• željeti podršku = to want support

So:

• podrška = nominative
• podršku = accusative

Because Croatian often omits subject pronouns when the verb ending already makes the subject clear.

• žele already means they want

So Croatian does not need to say oni or one here.

The subject is understood from the context: it is still Ana i Sara.

You could add a pronoun for emphasis, but normally it is unnecessary.

Both kad and kada mean when.

In sentences like this, they usually mean the same thing. The difference is mostly style:

• kad = shorter, very common in everyday speech
• kada = a bit fuller or more formal

So kad žele podršku and kada žele podršku are both correct.

Because šalju comes from the imperfective verb slati, which is the natural choice for habitual or repeated actions.

This sentence says:

• često šalju = they often send

That describes something they do regularly, not one single completed act.

By contrast, pošalju is from the perfective verb poslati, which usually focuses on a completed sending event.

So:

• često šalju = habitual, repeated
• pošalju = send off / complete the sending

Yes, Croatian word order is fairly flexible, but this version sounds neutral and natural:

• Ana i Sara često šalju jedna drugoj poruke kad žele podršku.

A rough structure is:

• subject: Ana i Sara
• adverb: često
• verb: šalju
• indirect object / reciprocal phrase: jedna drugoj
• direct object: poruke
• time clause: kad žele podršku

You can move things around for emphasis, but this order is a very normal one.

Because Croatian has no articles like English the, a, or an.

So Croatian simply says:

• poruke = messages / the messages / some messages
• podršku = support / the support

The exact meaning is understood from context.

In this sentence, English would naturally say messages and support, but Croatian does not need articles to express that.