Боязнь сцены беспокоит друга.

Because друга is the accusative singular of друг.

In this sentence, друг is the object of the verb беспокоит (worries / bothers), so it must go into the accusative case.

For masculine animate nouns in Russian, the accusative singular usually looks like the genitive singular:

• друг = nominative
• друга = accusative/genitive

So:

• друг = friend as the subject
• друга = friend as the direct object

That is why Боязнь сцены беспокоит друга means that the friend is being worried or bothered by stage fright.

Because of the cases.

• Боязнь is in the nominative case, which is the normal case for the subject.
• друга is in the accusative case, which marks the direct object.

So the structure is:

• Боязнь сцены = the subject
• беспокоит = the verb
• друга = the object

Even though English learners may want to read friend as the subject, Russian tells you who does what mainly through case endings, not just word order.

So this means:

• Stage fright worries the friend not
• The friend worries stage fright

Because боязнь often takes a noun in the genitive to show what the fear is about.

So:

• боязнь сцены = fear of the stage / stage fright

This is a common Russian pattern:

• страх темноты = fear of the dark
• боязнь высоты = fear of heights
• чувство юмора = sense of humor

Here, сцены is the genitive singular of сцена.

So literally, боязнь сцены is something like fear of the stage.

Formally, сцены can be either:

• genitive singular of сцена = of the stage
• nominative plural = stages

But in this sentence, it is clearly genitive singular, because it depends on боязнь.

So боязнь сцены is understood as:

• fear of the stage not
• fear stages

Context and grammar make the meaning clear.

Беспокоит means worries, bothers, or troubles.

It comes from the verb беспокоить.

In this sentence:

• Боязнь сцены беспокоит друга = Stage fright worries the friend / The friend is troubled by stage fright

Depending on context, English may translate it in slightly different ways:

• worries
• bothers
• troubles
• causes anxiety to

Russian often uses this verb with a thing as the subject and a person as the object:

• Меня беспокоит шум. = The noise is bothering me.
• Её беспокоит экзамен. = The exam is worrying her.

Because the subject is боязнь, which is singular.

Even though боязнь сцены is a two-word phrase, the grammatical head of the phrase is боязнь. That noun is singular, so the verb is singular too:

• боязнь → singular
• беспокоит → singular

If the subject were plural, the verb would also be plural:

• Проблемы беспокоят друга. = Problems worry the friend.

Yes. Боязнь is a feminine noun.

In Russian, nouns ending in -ь can be either masculine or feminine, so you often have to learn the gender separately.

For example:

• день = masculine
• словарь = masculine
• ночь = feminine
• боязнь = feminine

A useful clue here is the meaning and dictionary form, but ultimately this is something you memorize.

Yes. Russian word order is flexible because the cases show the grammatical roles.

For example, these are all possible:

• Боязнь сцены беспокоит друга.
• Друга беспокоит боязнь сцены.
• Беспокоит друга боязнь сцены.

They all keep roughly the same core meaning because:

• боязнь is still nominative
• друга is still accusative

What changes is the emphasis:

• starting with боязнь сцены highlights the cause
• starting with друга highlights the person affected

Russian has no articles, so друга can mean:

• a friend
• the friend
• sometimes even his/her friend, depending on context

You decide from the situation or surrounding text.

So Боязнь сцены беспокоит друга could be translated as:

• Stage fright worries a friend
• Stage fright worries the friend

If the broader context already identifies the friend, English usually uses the.

Yes, it is understandable and natural. It literally means fear of the stage.

Another very common expression is:

• страх сцены

Both can mean stage fright.

A learner should recognize both:

• боязнь сцены
• страх сцены

They are close in meaning, though страх can sound a bit stronger and more general as fear, while боязнь often has the sense of fearfulness or being afraid of something.

They are:

• боязнь = fear
• сцена = stage
• беспокоить = to worry / bother
• друг = friend

The forms in the sentence are:

• Боязнь = nominative singular
• сцены = genitive singular
• беспокоит = 3rd person singular present
• друга = accusative singular

So a learner can think of the sentence like this:

• боязнь
  • сцены = fear of the stage
• беспокоит = worries
• друга = the friend / a friend

That gives the overall structure: Fear of the stage worries the friend.