Why Serbian keeps saying that I go instead of to go
Russian has an infinitive and uses it constantly. Serbian has one too - and avoids it in exactly the situations where Russian reaches for it, replacing it with a small word, да, and a fully conjugated verb. This is not a quirk of Serbian alone; it is a shared feature of half a dozen unrelated Balkan languages.
A Russian speaker's first hundred sentences in Serbian tend to include the same small mistake, made with total confidence: reaching for the infinitive exactly where Russian would, and getting a sentence that a Serb understands but flags as slightly foreign. Хоћу отићи instead of Хоћу да одем. The mistake is worth pausing on, because it is not really about one verb - it is a whole grammatical system, and once you see the pattern it stops being a list of exceptions to memorise.
The pattern: да plus a conjugated verb
Where Russian chains a modal or phase verb directly onto an infinitive, as in хочу пойти (I want to go), colloquial Serbian, especially south of Belgrade and throughout Macedonia and Bulgaria, prefers a different shape entirely: the particle да followed by a verb conjugated for the same person as the subject of the first clause.
Хоћу да одем.
I want to go. (literally: I want that I go)
Могу да ти помогнем.
I can help you. (literally: I can that I help you)
Почела је да говори.
She started talking. (literally: she started that she talks)
Notice the second person doubles up inside a single clause: да ти помогнем repeats the subject's person on the second verb, something that would be redundant in Russian and is simply obligatory here. This is the part that feels most unnatural to a Russian speaker at first, and it is also the part that is completely regular once internalised - the second verb always agrees with whoever is doing the second action, which is not always the same person as the first verb's subject.
Желим да ми помогнеш.
I want you to help me. (literally: I want that you help me)
Russian needs «чтобы» here for exactly the same reason - the subject changes between the two verbs. The difference is that Russian only reaches for this heavier construction when the subjects differ; Serbian uses да whether they differ or not.
Not a Serbian quirk - a Balkan one
This is where the story gets bigger than one language. Bulgarian, Macedonian, Albanian, Romanian and Greek - none of them related to Serbian by descent, spanning three separate language families - have each independently lost or restricted the productive infinitive and replaced it with almost exactly this same construction: a subordinating particle plus a fully conjugated verb. Bulgarian and Macedonian have gone the furthest and lost the infinitive from the standard language entirely; Serbian keeps a living infinitive (you will meet it as the dictionary form of every verb, and it still appears after ће in the literary future) but avoids it colloquially in precisely the contexts described above.
Linguists call this shared cluster of features - infinitive loss among them - the Balkan sprachbund ("language union"): a set of grammatical traits that spread across a region by centuries of contact and bilingualism, cutting straight across the Slavic/Romance/Greek/Albanian family boundaries. It is one of the textbook examples in the field precisely because the languages involved are not related, which rules out common ancestry as the explanation and leaves only geography and contact.
The practical upshot
The safe default for a learner: after a modal verb («хтети», «моћи», «морати»), after a phase verb («почети», «престати»), and after a verb of wanting or asking that could take «чтобы» in Russian, reach for да + a conjugated verb, not the infinitive. The infinitive still exists and is not wrong, but the colloquial default has shifted, and да is the form you will hear far more often in an actual conversation in Belgrade.