Он боится одиночества.

Because бояться normally takes the genitive case, not the nominative/accusative.

• Dictionary form: одиночество = loneliness
• Genitive singular: одиночества

So:

• Он боится одиночества = correct
• Он боится одиночество = not standard Russian

This is something you simply have to learn with the verb: бояться + genitive.

Here it is genitive singular.

That can be confusing because одиночества can also look like a nominative plural form for some neuter nouns. But in this sentence, the verb бояться tells you it must be genitive.

So the logic is:

• бояться requires genitive
• therefore одиночества here means of loneliness, not lonelinesses

Also, since одиночество is an abstract noun, plural is unusual in normal speech anyway.

The dictionary form is бояться.

Боится is the 3rd person singular form, meaning he/she/it is afraid or fears.

Present-tense forms of бояться:

• я боюсь
• ты боишься
• он/она/оно боится
• мы боимся
• вы боитесь
• они боятся

So in your sentence:

• он = he
• боится = is afraid / fears

The -ся ending is a very common Russian verb ending, often called the reflexive marker.

In some verbs, it has a clearly reflexive meaning, like мыться = to wash oneself. But in бояться, you should think of -ся as just part of the verb itself. It does not literally mean to fear oneself.

So learn бояться as a whole word.

Боится is present tense and imperfective.

Here it describes a state or ongoing feeling, not a single completed action. Russian uses the present tense for things like general emotions and habits, just as English does:

• Он боится одиночества = He is afraid of loneliness / He fears loneliness

If you wanted the idea of becoming scared at a particular moment, Russian would often use a different verb, such as испугаться.

So:

• бояться = to be afraid, to fear
• испугаться = to get scared, to become frightened

Yes, sometimes.

Russian often drops subject pronouns when the subject is clear from context. So if everyone already knows who you are talking about, you could say simply:

• Боится одиночества.

But with no context, that sounds incomplete in English and can sound context-dependent in Russian too. The full sentence with он is the neutral, clear version.

The most neutral word order is:

• Он боится одиночества.

But Russian word order is more flexible than English, and changing it can change the focus or emphasis.

For example:

• Одиночества он боится. = It is loneliness that he is afraid of.
• Боится он одиночества. = He is afraid of loneliness, with extra focus on he is afraid

So the original order is the best basic pattern for learners.

A helpful approximate pronunciation is:

• боится → ba-EET-sa
• одиночества → a-dee-NO-chee-stva

Stress:

• бои́тся — stress on и
• одино́чества — stress on но́

One useful pronunciation note: although боится is spelled with -тся, in normal speech that ending sounds roughly like -ца.

Because Russian has no articles.

English says:

• a loneliness / the loneliness / just loneliness

Russian does not use separate words for a or the. The meaning comes from context.

So одиночества here simply means loneliness in whatever way fits the context. In this sentence, English also normally uses no article:

• He is afraid of loneliness.

Because after бояться, you usually need a noun or noun-like expression for the thing feared.

• одиночество = loneliness
• одинокий = lonely / lonely person, depending on form

So Он боится одиночества is natural because одиночество names the thing he fears.

If you want to express a similar idea another way, Russian might use:

• Он боится быть один. = He is afraid of being alone.

That is slightly different in nuance:

• боится одиночества = afraid of loneliness as a state/condition
• боится быть один = afraid to be alone / afraid of being alone