Learn Hardcore Croatian: Često moj brat i sestra mogu pomoći baki. - My brother and sister often can help grandmother.

Because the verb pomoći (to help) governs the dative case: you “help to someone.” The dative singular of baka is baki. By contrast, verbs that take a direct object use the accusative: Vidim baku (I see grandma), but Pomažem/Pomoći ću baki (I help/I will help grandma).

Why is the verb mogu and not može?

The subject is plural (moj brat i sestra = my brother and sister → they), so the verb must be 3rd person plural: mogu. If it were only the brother, you’d use singular: Moj brat može…

Should it be moj brat i moja sestra, or is moj brat i sestra OK?

Both are acceptable. In everyday Croatian, a single possessive (moj) before the first noun typically applies to both nouns, so moj brat i sestra is normally understood as “my brother and (my) sister.” Repeating the possessive (moj brat i moja sestra) makes it explicit and a bit more formal or careful.

Can I say moji brat i sestra?

No. The possessive must agree with the noun it directly modifies. Brat is masculine singular, so you need moj brat. If you want the possessive with both nouns, use each separately: moj brat i moja sestra.

Why is the second verb in the infinitive (pomoći) after mogu?

In standard Croatian, modals like moći are followed by the infinitive: mogu pomoći, ne mogu razumjeti, etc. Croatian generally does not use the da-construction with modals (see next question).

Is mogu da pomognu acceptable?

Not in standard Croatian; that’s characteristic of Serbian. Standard Croatian prefers the infinitive: mogu pomoći. If you want a finite clause in Croatian, you’d restructure the sentence differently.

Does pomoći fit with često, or should it be pomagati/pomažu?

Both are possible, with a nuance:

često mogu pomoći = they often are able to render help (repeated successful, bounded events; perfective).
More natural for habitual action is the imperfective present: Moj brat i sestra često pomažu baki (they often help grandma). You can also say često mogu pomagati, but in practice speakers often drop moći and use pomažu for habits.

Where can I put često in the sentence?

Word order is flexible. Common, natural options include:

Moj brat i sestra često mogu pomoći baki.
Često moj brat i sestra mogu pomoći baki.
Moj brat i sestra mogu često pomoći baki. (a bit more marked, stressing the frequency of their ability) All are grammatical; placement slightly shifts emphasis/prosody, not the core meaning.

Do I need a comma after Često at the start?

No. After a single short adverb like Često, a comma is not used: Često moj brat i sestra…

Can I replace baki with a pronoun? Where does it go?

Yes. The dative pronoun for “to her” is joj, which is a clitic and tends to appear in second position in the clause:

Moj brat i sestra joj često mogu pomoći.
Često joj moj brat i sestra mogu pomoći.

How would I say “to our grandma” or “to their own grandma”?

“to our grandma”: našoj baki (dative singular)
“to their own grandma”: svojoj baki (reflexive possessive, preferred when the possessor is the subject in 3rd person) Example: Moj brat i sestra često mogu pomoći svojoj baki.

What’s the agreement rule for a compound subject like brat i sestra?

With subjects joined by i (and), the verb is plural: brat i sestra mogu. With ili (or), agreement can be singular if the meaning is exclusive and the nouns are both singular: Brat ili sestra može…

Is there any article missing (like “the”)?

Croatian has no articles. Possessives (like moj) are adjectives, not articles, and you rely on context for definiteness.

How do I pronounce and distinguish č (in Često) and ć (in pomoći)?

Both are voiceless affricates, but:

č is harder/longer (like English “ch” in “charm”).
ć is softer/shorter, produced further forward in the mouth. Spelling matters: pomoći with ć means “to help,” while pomaći (with a) is a different verb meaning “to move something a bit.”