Я позвонил подруге, чтобы успокоить её перед экзаменом.

Позвонил is the perfective past form of позвонить, which usually means to make a call / to call once and emphasizes a completed action.

• Я позвонил подруге = I called my friend / I gave my friend a call
• Я звонил подруге would use the imperfective verb звонить, which often means:
  • I was calling my friend
  • I used to call my friend
  • I called my friend with more focus on the process or repeated action, depending on context

In this sentence, the speaker made one completed phone call for a purpose, so позвонил is the natural choice.

That is the normal marker for the past tense in Russian.

The infinitive is позвонить. To form the past tense, Russian usually replaces the infinitive ending with a past marker:

• позвонить → позвонил = he/I (male) called
• позвонила = she/I (female) called
• позвонило = it called
• позвонили = they called

So if the speaker is male, Я позвонил is correct. If the speaker is female, it would be Я позвонила.

Because the verb позвонить takes the dative case, not the accusative.

You ask:

• позвонить кому? = to call whom?

So:

• подруга = nominative
• подруге = dative

That is why:

• Я позвонил подруге = I called my female friend

This is different from English, where call someone uses a direct object. In Russian, with позвонить, the person called is treated more like to call to someone grammatically.

Подруга means female friend.

So:

• друг = male friend
• подруга = female friend

In this sentence, подруге is the dative singular form of подруга.

A native English speaker may also notice that Russian often makes the gender of the friend explicit in a way English usually does not.

Because чтобы introduces a purpose clause.

The structure is:

• Я позвонил подруге, чтобы успокоить её...
• I called my friend in order to calm her...

In Russian, clauses introduced by чтобы are normally separated by a comma.

So the comma here is standard punctuation.

Here чтобы means so that, in order to, or to.

In this sentence:

• Я позвонил подруге, чтобы успокоить её...
• literally: I called my friend, in order to calm her...

It introduces the speaker’s purpose.

A very common pattern is:

• [main action], чтобы + infinitive
• did X in order to do Y

Examples:

• Я пришёл, чтобы помочь. = I came to help.
• Она открыла окно, чтобы проветрить комнату. = She opened the window to air out the room.

Успокоить is the perfective infinitive, meaning to calm someone down completely / successfully.

Compare:

• успокаивать = imperfective, focusing on the process: to be calming / to calm repeatedly
• успокоить = perfective, focusing on the result: to calm

In this sentence, the goal of the phone call is to achieve a result, so успокоить is the better choice:

• чтобы успокоить её = to calm her down

If you used успокаивать, it would sound more like focusing on the activity rather than the intended result.

Because её is the object of успокоить.

The sentence has two different relationships:

1. позвонил подруге = called my friend

   • подруге is the person who received the phone call

2. успокоить её = calm her

   • её is the person being calmed

Even though both refer to the same person, Russian normally includes the pronoun here because успокоить needs an object.

Without её, the sentence would feel incomplete in most contexts.

Because успокоить takes a direct object, so the pronoun must be in the accusative case.

For она (she), the accusative/genitive form is её:

• она = she
• её = her

So:

• успокоить её = to calm her

A useful thing to know is that её can also be a possessive word meaning her, but here it is clearly the pronoun her.

Because the preposition перед requires the instrumental case when it means before in time or in front of in space.

You ask:

• перед чем? = before what?

So:

• экзамен = nominative
• экзаменом = instrumental

That gives:

• перед экзаменом = before the exam

This is a very common case pattern to memorize:

• перед + instrumental

Here it clearly means before the exam in a time sense.

So:

• перед экзаменом = before the exam

The same preposition перед can also mean in front of, but context tells you which meaning is intended.

For example:

• Она стояла перед домом. = She was standing in front of the house.
• Он волновался перед экзаменом. = He was nervous before the exam.

In your sentence, calming someone down fits the time meaning.

Yes, Russian often can leave it out:

• Позвонил подруге, чтобы успокоить её перед экзаменом.

Because the verb form already shows past tense and gender, Russian does not always need the subject pronoun.

However, Я is still perfectly natural. It can be used:

• for clarity
• for emphasis
• because the speaker simply chooses to include it

So both versions are possible, but Я позвонил... is completely normal.

Not completely. Russian word order is fairly flexible, though some orders sound more neutral than others.

The given sentence:

• Я позвонил подруге, чтобы успокоить её перед экзаменом.

is neutral and natural.

You could rearrange parts for emphasis, for example:

• Перед экзаменом я позвонил подруге, чтобы успокоить её.
  • emphasizes before the exam
• Подруге я позвонил, чтобы успокоить её перед экзаменом.
  • emphasizes my friend

But the original order is probably the best default version for a learner.

No, not naturally.

Because of the structure, the most natural reading is:

• the speaker called the friend
• in order to calm that same friend

So подруге and её are understood as the same person.

If Russian wanted to make it clear that the speaker called one person in order to calm a different person, it would usually need extra wording to avoid ambiguity.