Моё сердце быстро бьётся, когда я нервничаю.

In Russian, possessive pronouns agree in gender, number, and case with the noun they modify.

• сердце is grammatically neuter (you can see this from the typical neuter ending -е in the nominative singular).
• Therefore you must use the neuter form of “my”: моё.
• мой is masculine, so it works with masculine nouns like мой стол (my table).
• The full set in the nominative singular is:
  • мой стол (masculine)
  • моя книга (feminine)
  • моё сердце (neuter)
  • мои друзья (plural)

So моё сердце is simply grammatical agreement: neuter pronoun with a neuter noun.

бьётся is:

• Present tense
• 3rd person singular
• Imperfective aspect
• Reflexive form of the verb биться

Russian does not have a special “continuous” form like English “is beating”. The simple present in Russian covers:

• General truths / regular actions: Моё сердце быстро бьётся, когда я нервничаю.
  → My heart beats fast / is beating fast when I’m nervous.
• Actions happening right now: Сейчас моё сердце быстро бьётся.
  → Right now my heart is beating fast.

So бьётся here can be translated either as “beats” or “is beating” depending on the English style you prefer.

The -ся / -сь ending marks a reflexive / “middle” verb in Russian. It often makes the verb:

• Reflexive: the subject acts on itself.
• Or intransitive / “middle voice”: an action happens without a clear direct object.

Compare:

• бить – “to beat, to hit (something/someone)”
  
  • Он бьёт мяч. – He hits the ball. (direct object)
• биться – “to beat (itself), to struggle, to bang, to fight”
  
  • Сердце бьётся. – The heart beats (itself).
  • Птица бьётся в клетке. – The bird is flapping / beating against the cage.

For the heart, Russian always uses биться:
Моё сердце быстро бьётся.
Saying сердце бьёт sounds wrong to native speakers.

Both быстро бьётся and бьётся быстро are grammatically correct.

• The most neutral order is usually Adverb + Verb:
  Моё сердце быстро бьётся.
• Putting the adverb after the verb (бьётся быстро) can add a bit of emphasis or stylistic flavor, sometimes sounding more expressive or poetic.

Meaning-wise there is no real difference here; it’s mostly about style and rhythm. In everyday speech, быстро бьётся is more typical.

сердце is in the nominative singular case.

• It is the subject of the sentence: it’s the thing that is doing the action (it is beating).
• The basic pattern is:
  • Кто? / Что? (Who? / What?) → nominative
  • Что делает? (What does it do?) → verb

So:

• Что (что именно) бьётся? – сердце.
  Therefore, сердце is nominative.

In Russian, когда introduces a subordinate clause of time. The rule is:

• Any full subordinate clause introduced by когда, потому что, если, чтобы, хотя, etc. is separated from the main clause by a comma.

Here:

• Main clause: Моё сердце быстро бьётся
• Subordinate clause: когда я нервничаю

So you must write:
Моё сердце быстро бьётся, когда я нервничаю.

This is different from English, where a comma before “when” is sometimes optional.

Yes. The word order is flexible:

• Когда я нервничаю, моё сердце быстро бьётся.
• Моё сердце быстро бьётся, когда я нервничаю.

Both are correct. The comma just stays between the two clauses.

Nuance:

• Starting with Когда я нервничаю… slightly emphasizes the condition/situation.
• Starting with Моё сердце… emphasizes the heart’s reaction.

But in everyday speech, the difference is very small; both sound natural.

In Russian, a temporary emotional state is often expressed with a verb, not an adjective:

• я нервничаю – I’m (feeling) nervous / I’m getting nervous.
• я волнуюсь – I’m worried / nervous / excited.

The adjective нервный describes a more permanent characteristic or something “relating to nerves”:

• Он нервный человек. – He is a nervous (high-strung) person.
• Нервная система. – The nervous system.

So for “when I’m (feeling) nervous” in a given situation, когда я нервничаю is the natural choice.

Both can translate as “I’m nervous”, but there are nuances:

• нервничать
  • Focuses on nervousness, anxiety, being on edge.
  • Often about stress, fear of something going wrong.
• волноваться
  • Broader: can mean worry, be anxious, but also be emotionally stirred, even in a positive way.
  • Я волнуюсь перед экзаменом. – I’m worried before the exam.
  • Я волнуюсь перед концертом. – I’m nervous/excited before the concert.

In your sentence, когда я нервничаю = “when I’m (feeling) nervous / when I get nervous”, with a clear nervous/anxious connotation.

You can say that, and it will be understood, but:

• Сердце быстро бьётся, когда я нервничаю.
  Sounds a bit more impersonal or literary; “the heart beats fast when I’m nervous” (whose heart is understood from the context).
• Моё сердце быстро бьётся, когда я нервничаю.
  Sounds clearly personal and is very natural.
• Another very natural everyday option is:
  У меня быстро бьётся сердце, когда я нервничаю.
  Literally: “With me the heart beats fast…”, but it means “My heart beats fast when I’m nervous.”

All three are grammatically correct; the original sentence is a straightforward, personal version.

Stress (accent) in each word:

• моё́ – stress on ё (always stressed).
• се́рдце – stress on the first syllable: се́рд-.
• бы́стро – stress on бы́-.
• бьё́тся – stress on ё.
• когда́ – stress on -да́.
• я – only one syllable, so no issue.
• не́рвничаю – stress on не́р- (first syllable).

Put together (stressed vowels marked):
моё́ се́рдце бы́стро бьё́тся, когда́ я не́рвничаю.

Pronunciation notes:

• бьётся: the ь makes б soft (like “by” in English “byo”), and ё is pronounced “yo”.
• сердце: the д is mostly silent; it sounds like сéр-це.
• нервничаю: there is a consonant cluster рвн, but native speakers pronounce it smoothly: НЕ́рв-ни-ча-ю.

They are related but different parts of speech:

• быстро – an adverb (“quickly, fast”). It answers “How?”
  
  • Сердце быстро бьётся. – The heart beats fast.
• быстрый – an adjective (“fast, quick”). It answers “What kind of?”
  
  • быстрый поезд – a fast train.
  • быстрый шаг – a quick step.

So:

• Use быстро with verbs:
  Он быстро говорит. – He speaks quickly.
• Use быстрый with nouns:
  быстрый разговор – a fast conversation.

Yes, but the meaning changes slightly:

• быстро бьётся – “beats quickly / fast” → emphasizes speed.
• сильно бьётся – “beats strongly / hard” → emphasizes intensity / force.

So:

• Моё сердце быстро бьётся, когда я нервничаю.
  → My heart beats faster when I’m nervous.
• Моё сердце сильно бьётся, когда я нервничаю.
  → My heart is pounding / beating hard when I’m nervous.

Both are natural; choose the one that matches the nuance you want.