The se that isn't always there: reflexive verbs that don't match Russian's -sya
Serbian marks a reflexive verb with a free-standing се instead of a suffix, and most of the time it lines up with Russian's -ся closely enough to stop thinking about it. It is the exceptions - a handful of everyday verbs reflexive in one language and plain in the other - that make a fluent-sounding sentence suddenly wrong.
Serbian and Russian both mark a reflexive verb by attaching something to it - Russian glues -ся onto the end of the verb itself, Serbian leaves се as its own free-standing word, moving around the sentence the way any other clitic does. Structurally that is a real difference, but it rarely trips anyone up, because for the huge majority of verbs the two languages agree on which ones need the marker at all. Смејати се is reflexive, смеяться is reflexive, done. The genuinely useful question is not how the marker attaches - it is which verbs get one.
Where the agreement holds
Worth stating first, because it is the majority case and it is easy to forget once you start hunting for exceptions: dozens of common verbs are reflexive in both languages, for the same reason. Fear, hope and effort are the clearest family - the thing being feared, hoped for or worked toward sits outside the verb, and the verb itself marks that its subject is the one doing the fearing, hoping or trying.
Бојим се мрака.
I'm afraid of the dark.
Бојати се / бояться - reflexive in both languages, genitive object in both. One of the cleanest matches in the whole verb system.
Надам се да ће бити боље.
I hope it'll be better.
Надати се / надеяться - same marking, same idea.
Where Serbian adds се and Russian stays plain
The first kind of mismatch is a Serbian verb quietly reflexive where its Russian counterpart is not. Играти се needs its се for unstructured play - the kind with no rules and no opponent - while Russian играть never takes -ся for the same activity. Joking around is the same story: шалити се needs its се, шутить has never needed a marker of any kind.
| Serbian | Russian |
|---|---|
| играти се | играть |
| шалити се | шутить |
Where Russian adds -ся and Serbian stays plain
The mismatch runs the other way just as often. Russian ошибаться carries its -ся everywhere the Serbian cognate грешити carries nothing - plain, unmarked, no clitic to place. Остати needs nothing at all for the plain idea of staying put; Russian marks that identical meaning with the reflexive остаться (or its imperfective оставаться) and has no unmarked alternative for it at all. Neither verb is irregular in its own language - they are simply built on different assumptions about which situations count as acting on yourself.
| Serbian | Russian |
|---|---|
| остати | остаться |
| грешити | ошибаться |
There is no shortcut that works every time, but the surest test is to check what sits on the other side of the verb: memorise the verb together with its marker - or lack of one - rather than trusting the meaning to carry the grammar along with it. The mismatch list is short, but it is not zero, and the difference is invisible when you read, only audible when you build the sentence yourself.
Мали Марко се игра у дворишту.
Little Marko is playing in the yard.
Игра се carries се for exactly this meaning; the Russian translation carries nothing at all.
Остао сам код куће цео дан.
I stayed home all day.
Остао сам carries no marker at all; the Russian остался cannot drop its -ся for this meaning.
None of this touches the grammar most learners actually struggle with - where се sits in the sentence, a word-order question this blog has already covered. This is a smaller, quieter problem: a fixed, short list of verbs where fluency in one language actively misleads the ear in the other, in both directions at once.