Он боится снова проиграть.

Because the dictionary form is бояться, which means to be afraid. This is a verb that normally appears with -ся.

In this sentence, боится is:

• 3rd person singular
• present tense
• from бояться

So:

• я боюсь = I am afraid
• он боится = he is afraid

You should learn бояться as a whole verb, not as боить + ся.

In бояться, -ся is part of the verb. Historically it is related to reflexive meaning, but in modern Russian, with many verbs, it is best to treat it as simply part of the dictionary form.

So бояться is the normal verb for to be afraid. You do not usually remove -ся and use the rest by itself.

After бояться, Russian often uses an infinitive to say what someone is afraid to do or afraid may happen as a result of their action.

So:

• Он боится проиграть = He is afraid to lose / He is afraid of losing

This is a very natural Russian structure:

• бояться + infinitive

Other examples:

• Я боюсь опоздать = I am afraid of being late / I am afraid I’ll be late
• Она боится упасть = She is afraid of falling

This is about aspect.

• проиграть = perfective
• проигрывать = imperfective

In this sentence, проиграть is used because the speaker is talking about one complete possible loss, not an ongoing or repeated process.

So Он боится снова проиграть suggests:

• he is afraid that he will lose once again
• a specific future defeat is being imagined

If you said боится снова проигрывать, it would sound more like:

• being afraid of repeatedly losing
• being afraid of a habit or ongoing pattern of losing

So the perfective проиграть fits better here.

Because the sentence describes his current emotional state.

• Он боится = He is afraid now
• проиграть = to lose, as a possible future action

So Russian uses present tense for the fear itself, even if the thing feared has not happened yet.

This is similar to English:

• He is afraid he will lose again
• the fear is present, the loss is future

Снова means again.

In this sentence, it modifies проиграть, so the meaning is:

• he is afraid to lose again
• he is afraid of another defeat

So the idea is not just that he is afraid, but that he fears a repetition of an earlier loss.

In Он боится снова проиграть, the most natural reading is:

• снова goes with проиграть
• he is afraid of losing again

If you wanted to say that the fear itself has returned, Russian would more naturally say:

• Он снова боится проиграть = He is again afraid of losing

So word order can change what снова applies to.

Yes, in many contexts you can.

• Он боится снова проиграть
• Он боится опять проиграть

Both can mean He is afraid of losing again.

A small nuance:

• снова often sounds a bit more neutral
• опять can sometimes sound more conversational, and in some contexts it may carry a feeling like yet again

But in this sentence, both are possible.

Yes. That is also natural Russian.

Compare:

• Он боится снова проиграть
• Он боится, что снова проиграет

Both mean roughly He is afraid he will lose again.

The difference is mainly structural:

1. боится + infinitive

   • more compact
   • often used when talking about what someone is afraid to do or afraid may happen

2. боится, что + finite verb

   • more explicit
   • often closer to English is afraid that...

So both are correct; the second one spells out the feared event more directly.

Yes. When бояться is followed by a noun or pronoun, it usually takes the genitive:

• бояться темноты = to be afraid of the dark
• бояться собак = to be afraid of dogs

But in your sentence, бояться is followed by an infinitive:

• боится проиграть

An infinitive is not a noun, so there is no noun case ending to look for here. It is simply a verbal complement after бояться.

Yes. Проиграть often means to lose in a competition, game, match, election, argument, or similar situation.

So in this sentence, without more context, it could mean:

• lose a game
• lose a match
• lose a competition
• suffer another defeat

The exact kind of losing depends on context.

It is pronounced approximately ba-EE-tsa.

A few useful points:

• the stress is on the second syllable: бои́тся
• the ending -тся in speech sounds like -ца
• so learners often hear it as something like баится

This is one of those places where Russian spelling and pronunciation are not perfectly obvious at first, so it is worth memorizing the spoken form.