Bojim se psa.

Because the full verb is bojati se, which means to be afraid or to fear. The se is part of the verb and normally cannot be dropped.

So:

• bojati se = to be afraid
• bojim se = I am afraid

This is one of many Serbian verbs that are used with se.

Bojim se is the 1st person singular present tense form of bojati se.

It means:

• I am afraid
• I fear

A few present-tense forms are:

• ja se bojim = I am afraid
• ti se bojiš = you are afraid
• on/ona se boji = he/she is afraid
• mi se bojimo = we are afraid
• vi se bojite = you are afraid
• oni se boje = they are afraid

In actual Serbian, the pronoun ja is usually omitted unless you want emphasis, so Bojim se psa is perfectly normal.

Because bojati se takes the genitive case, not the nominative.

• pas = dog, in the dictionary/basic form
• psa = of the dog / dog, in the genitive singular

So:

• Bojim se psa. = I am afraid of the dog / a dog

This is something learners usually just need to memorize: bojati se + genitive.

That is simply how this verb works in Serbian. Some verbs require a specific case, and bojati se is one of the verbs that takes genitive.

So you learn it as a pattern:

• bojati se nekoga / nečega = to be afraid of someone / something

Examples:

• Bojim se mraka. = I’m afraid of the dark.
• Bojim se visine. = I’m afraid of heights.
• Bojim se psa. = I’m afraid of the dog / a dog.

Serbian has no articles like the and a, so psa can mean different things depending on context:

• the dog
• a dog
• sometimes even dogs in a more general sense, though other wording may be clearer

Out of context, Bojim se psa most naturally suggests I’m afraid of the dog or I’m afraid of a dog.

Context decides the exact meaning.

It is understandable, but the most natural neutral order is:

• Bojim se psa.

You can also say:

• Ja se bojim psa.

If you include ja, it is usually for emphasis, contrast, or clarity.

Also note the position of se: it typically comes early in the sentence, often after the first stressed word. That is why Ja se bojim psa sounds natural, while Ja bojim se psa does not.

Yes. Serbian word order is fairly flexible, but different orders change the emphasis.

Neutral:

• Bojim se psa.

Emphatic or contrastive:

• Psa se bojim. = It’s the dog that I’m afraid of.
• Ja se bojim psa. = I’m the one who is afraid of the dog.

So the basic meaning stays the same, but the focus shifts.

No, not in standard Serbian.

With bojati se, you normally do not use a preposition like od. You use the genitive directly:

• bojim se psa
• bojim se mraka
• bojim se njega

So the correct pattern is:

• bojati se + genitive

Yes. A very common synonym is plašiti se.

So you can also say:

• Plašim se psa.

This also means I am afraid of the dog / a dog.

For many everyday situations, bojim se and plašim se are close in meaning, though nuances can vary depending on context and speaker.

You keep bojim se and put the next word into the genitive.

Examples with nouns:

• Bojim se čoveka. = I am afraid of the man.
• Bojim se deteta. = I am afraid of the child.
• Bojim se vode. = I am afraid of water.

Examples with pronouns:

• Bojim ga se. = I am afraid of him / it.
• Bojim je se. = I am afraid of her.
• Bojim ih se. = I am afraid of them.

These pronoun patterns can feel tricky at first, but the key idea is still the same: bojati se requires a special structure and keeps se.

Yes. In Serbian, the present tense here naturally expresses a current state.

So Bojim se psa can correspond to:

• I am afraid of the dog
• I fear the dog

In normal English, I am afraid of the dog is usually the more natural translation, but Serbian uses the simple present form bojim se for that idea.