Learn Hardcore Russian: Мой брат боится выступать перед публикой. - My brother is afraid to perform in front of an audience.

Many Russian verbs are reflexive, marked by -ся / -сь at the end. Historically this comes from the reflexive pronoun себя (oneself), but in modern Russian it’s just part of the verb.

бояться always has -ся.
There is no non‑reflexive form боить in modern Russian.

Conjugation (present tense):

я бою́сь – I am afraid
ты бои́шься – you are afraid
он / она бои́тся – he / she is afraid
мы бои́мся – we are afraid
вы бои́тесь – you (pl./formal) are afraid
они боя́тся – they are afraid

So Мой брат боится… is the only correct form here; боит is simply wrong.

Why is it боится выступать, not something like боится выступления?

Russian бояться can be used in two main ways:

бояться + genitive noun – to be afraid of something
- Он боится собак. – He is afraid of dogs.
- Она боится темноты. – She is afraid of the dark.
бояться + infinitive – to be afraid to do something
- Он боится плавать. – He is afraid to swim.
- Мой брат боится выступать. – My brother is afraid to perform / speak.

So:

Мой брат боится выступать = He is afraid to perform / to speak (the action).
Мой брат боится выступления = He is afraid of the performance (the event itself, as a thing).

Both are grammatically correct, but the sentence you gave focuses on the action of speaking rather than the event as an object.

Why is выступать in the infinitive? How does that work after боится?

After бояться, using an infinitive is the normal way to say “to be afraid to do something”.

Structure:

кто-то боится + infinitive

Examples:

Я боюсь летать. – I’m afraid to fly.
Она боится ошибиться. – She is afraid of making a mistake.
Мой брат боится выступать. – My brother is afraid to perform / speak (in public).

This infinitive can be translated into English either as “to do” or “of doing” depending on what sounds more natural:

боится выступать → “is afraid to speak / perform” or “is afraid of speaking / performing”.

Why is it выступать (imperfective), not выступить (perfective)?

Aspect in Russian is important:

выступать – imperfective: process, repeated / general actions.
выступить – perfective: one complete act, usually a specific event.

In Мой брат боится выступать перед публикой, the idea is general:

“He is (in general) afraid of speaking in public / of public speaking.”

If you say:

Мой брат боится выступить перед публикой.

you usually mean a specific upcoming performance:

“My brother is afraid (to go through with) giving this speech / making this appearance.”

So the imperfective выступать fits better for a general, habitual fear.

What exactly does выступать mean here? Is it just “to speak”?

Выступать is broader than just говорить (“to talk, to speak”).

выступать = to perform, to appear as a speaker or performer, to give a speech, a talk, a performance.
говорить = to speak, to talk (in general).

In the context выступать перед публикой, it normally means:

to give a speech,
to present,
to perform on stage (music, theatre, etc.).

So the sentence suggests he is afraid of public performances / public speaking, not just having a casual conversation.

What case is публикой, and why do we use that form after перед?

Публикой is in the instrumental case.

The preposition перед (“in front of, before”) normally takes the instrumental:

перед домо́м – in front of the house
перед школо́й – in front of the school
перед друзья́ми – in front of friends
перед публи́кой – in front of the public / the audience

The noun публика declines like this (singular):

Nominative: публика – the public
Genitive: публики
Dative: публике
Accusative: публику
Instrumental: публикой
Prepositional: публике

So after перед you must use публикой, not публика or публику.

Could I say на публике instead of перед публикой? What’s the difference?

Both are possible, but they sound a bit different.

перед публикой – literally “in front of the public / audience”;
strongly suggests a performance or speech situation.
- выступать перед публикой is a very natural collocation.
на публике – literally “in public, in front of other people”;
more general: any action done where people can see you.
- Он не плачет на публике. – He doesn’t cry in public.
- Она стесняется на публике. – She is shy in public.

Your sentence:

Мой брат боится выступать перед публикой. – focuses on public performances.
Мой брат боится выступать на публике. – understandable and okay, but slightly less idiomatic; it sounds more like “afraid of doing things when people are watching” rather than the standard fixed phrase for performing.

Where does the stress fall in Мой брат боится выступать перед публикой?

Stresses (marked with an acute accent):

Мой брат бои́тся выступа́ть пе́ред пу́бликой.

Word by word:

мой – one syllable, stress on the only vowel
брат – one syllable, stress on the only vowel
бои́тся – stress on и: бои́тся
выступа́ть – stress on final а́: выступа́ть
пе́ред – stress on first е́: пе́ред
пу́бликой – stress on у́: пу́бликой

So the rhythm is:
Мой брат бои́тся выступа́ть пе́ред пу́бликой.

Can I change the word order, for example Мой брат перед публикой боится выступать?

Russian word order is flexible, so several variants are grammatically possible:

Мой брат боится выступать перед публикой. – neutral, most natural.
Мой брат перед публикой боится выступать. – possible, but sounds a bit marked / less natural; it slightly emphasizes перед публикой.
Боится мой брат выступать перед публикой. – puts emphasis on боится (“It’s that he’s afraid to speak in public”).
Мой брат боится перед публикой выступать. – also possible, here перед публикой is highlighted more.

For learners, it’s safest (and most typical) to stick to:

Мой брат боится выступать перед публикой.

How do I make this sentence negative: “My brother is not afraid to speak in public”?

Use не before the verb боится:

Мой брат не боится выступать перед публикой.
– My brother is not afraid to speak in public.

Position:

Subject + не
- verb + infinitive + rest of sentence

Is боится here “present tense right now” or “usually / generally is afraid”?

In Russian, the present tense is used both for:

A state right now, and
A general / habitual state.

So Мой брат боится выступать перед публикой most naturally means:

“My brother (generally) is afraid of speaking in public / has stage fright.”

If you wanted to stress a very specific moment, you would usually add context:

Сейчас мой брат боится выступать перед публикой.
“Right now my brother is afraid to speak in public.”

Would the verb change if it were “My sister is afraid to speak in public”?

Only the possessive pronoun changes; the verb боится stays the same.

Мой брат боится выступать перед публикой. – My brother is afraid to speak in public.
Моя сестра боится выступать перед публикой. – My sister is afraid to speak in public.

In the present tense, Russian verbs do not change form for masculine vs. feminine in the 3rd person singular; they only change for person and number. Gender shows up in:

pronouns (он / она)
adjectives and possessives (мой / моя / моё / мои)

So:

он боится – he is afraid
она боится – she is afraid

How could I say more explicitly “My brother has stage fright” in Russian?

The original sentence already conveys that idea quite naturally:

Мой брат боится выступать перед публикой.
– My brother is afraid of speaking in public / has stage fright.

If you want to mention “stage fright” as a noun, you can say, for example:

У моего брата страх сцены. – My brother has stage fright.
У него сильный страх сцены. – He has a strong fear of the stage.

But in everyday speech, people often just use бояться + infinitive or a simple noun:

Он боится сцены. – He is afraid of the stage.
Он боится сцены, боится выступать перед публикой. – He has stage fright, he’s afraid of performing in front of an audience.