Ja često šaljem poruke sestri.

Yes. In Serbian, subject pronouns are often omitted because the verb ending already shows who is doing the action.

So:

Često šaljem poruke sestri.

already clearly means I often send messages to my sister because šaljem is a 1st person singular form.

Using ja is still correct, but it usually adds a bit of emphasis, contrast, or clarity:

• Ja često šaljem poruke sestri, a on retko. = I often send messages to my sister, but he rarely does.

Često means often.

Its position here is natural and neutral:

• Ja često šaljem poruke sestri.

But Serbian word order is flexible, so you can move it around depending on emphasis:

• Često šaljem poruke sestri.
• Poruke često šaljem sestri.

The original order is a very normal way to say it.

Šaljem is the 1st person singular present tense form of the verb slati, meaning to send.

So:

• slati = to send
• šaljem = I send / I am sending

The change from sl- to šalj- is just part of how this verb is conjugated, so it is something learners usually just memorize:

• ja šaljem
• ti šalješ
• on/ona šalje

Because poruke is the form used here for messages.

The basic noun is:

• poruka = message

In this sentence it is the direct object, and it is plural, so we get:

• poruke = messages

More specifically, this is accusative plural. For many feminine nouns ending in -a, the nominative plural and accusative plural look the same, so poruke can mean messages both as a subject and as an object depending on context.

If it were singular, you would say:

• šaljem poruku = I send a message / I am sending a message

Because sestri is the dative singular form of sestra.

The basic noun is:

• sestra = sister

But after verbs like give, send, show, the person who receives something is usually put in the dative:

• sestri = to the sister / to my sister

So in this sentence:

• poruke = what is being sent
• sestri = who receives them

Because Serbian often uses the dative case by itself to express the recipient, without a preposition.

English needs:

• to my sister

Serbian can simply use:

• sestri

So the idea of to is built into the case ending here.

Serbian often leaves out possessive words like my when the meaning is clear from context, especially with family relationships.

So:

• sestri can be understood as to my sister if the context already makes that clear.

If you want to say it more explicitly, you could say:

• mojoj sestri = to my sister
• svojoj sestri = to my own sister

Also, Serbian has no articles like a or the, so context does a lot of work.

With često in the sentence, the natural meaning is habitual or repeated:

• I often send messages to my sister.

So this is not mainly about one action happening right now. It describes something that happens regularly.

Serbian present tense can express both:

• something happening now
• something that happens generally or repeatedly

Here, često makes the repeated meaning clear.

Because šaljem comes from an imperfective verb, and imperfective is the normal choice for:

• repeated actions
• habits
• ongoing actions

Since često means often, the sentence describes a repeated habit, so šaljem fits naturally.

A perfective form such as pošaljem usually focuses more on a completed sending event, not just the general habit. So for this sentence, šaljem is the expected choice.

Yes. Serbian word order is much more flexible than English word order.

These are all possible, though they may sound slightly different in emphasis:

• Ja često šaljem poruke sestri.
• Često šaljem poruke sestri.
• Sestri često šaljem poruke.
• Poruke često šaljem sestri.

The original sentence is a neutral, natural version. Changing the order usually changes focus or emphasis, not the core meaning.