Questions & Answers about Sutra ću napisati pismo baki.
Ću is the future tense auxiliary, a short (clitic) form of the verb htjeti (hoću = I want / I will).
In this sentence, ću + infinitive (napisati) forms the Future I tense:
- ja ću napisati = I will write
In Croatian, ću is a clitic, so it normally stands in the second position in the clause, after the first stressed word or phrase:
- Sutra ću napisati pismo baki. – sutra is the first stressed word, so ću comes right after it.
- Ja ću sutra napisati pismo. – here ja is first, so ću comes right after ja.
So its position is not random: it follows the "first word/phrase" rule and stays close to the main verb (napisati).