Serbian's future tense has two correct spellings - and one that only looks like a third
„Радићу.“ and „Ја ћу радити.“ mean exactly the same thing, and both are completely correct - a fact that trips up learners who assume one of them has to be a mistake. The real trap sits one step further: a word order that looks like a natural blend of the two and is neither.
A learner who has just met Futur I typically reacts to seeing two different spellings for the same sentence with suspicion, as if the lesson made a typo. It did not. „Радићу.“ fuses the infinitive and the auxiliary into one written word; „Ја ћу радити.“ keeps them apart. Both are the standard language, not a formal-versus-casual pair, and picking between them is closer to choosing a word order than choosing a grammar rule.
The fusion rule, mechanically
When the infinitive comes first, drop its final -и and attach the short хтети ending directly: радити loses its last letter and gains -ћу/-ћеш/-ће…, landing on радићу. This is not optional spelling flexibility - writing the two as separate words in this order, „Радити ћу.“, is simply wrong, even though every other piece of that sentence is correct Serbian.
Спремићу вечеру до седам.
I'll have dinner ready by seven.
Fused: спремити loses its final -и before the short auxiliary attaches.
Она ће позвати такси.
She'll call a taxi.
Unfused: auxiliary first, infinitive stays whole and separate.
The infinitives that never fuse
Every infinitive ending in -ћи - ићи (to go), рећи (to say), and everything built on them like доћи or наћи - refuses fusion entirely, in either direction. The future of these verbs is always two separate words, auxiliary and infinitive, with nothing to memorize beyond that one exception: „Ја ћу отићи.“, never a fused form squeezed out of it.
| Infinitive first (fused) | Auxiliary first (separate) |
|---|---|
| радићу | ја ћу радити |
| читаћеш | ти ћеш читати |
| путоваћемо | ми ћемо путовати |
The colloquial third option, and why it isn't a third spelling of the same thing
Everyday speech has a third, extremely common way to talk about the future: хоћу/хоћеш… plus да plus a present-tense verb, as in „Хоћу да радим.“ It is worth recognizing, and it is not a third spelling of Futur I - it is a different construction entirely, built on the present tense rather than the infinitive, and it belongs to the same да plus conjugated-verb pattern this blog has covered as a Balkan-wide feature (linked below). Do not try to fuse it the way the standard future fuses - хоћу да радим never has a fused form at all.
Three patterns, three questions to check: if the infinitive comes first, fuse it - unless it ends in -ћи. If ћу/ћеш/ће… comes first, leave the infinitive whole. And if you hear да plus a conjugated verb, you are not looking at Futur I at all, so the fusion question does not even apply.