Перед экзаменом я глубоко вздохнул и вошёл в класс.

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

So:

• экзамен = nominative
• экзаменом = instrumental

In this sentence, перед экзаменом means before the exam.

Compare:

• перед домом = in front of the house
• перед встречей = before the meeting

Russian often uses case endings where English uses word order or separate function words.

Here it means before the exam in time.

The preposition перед can mean both:

• in front of something physically
• before something in time

The context tells you which meaning is intended. Since an exam is an event, перед экзаменом is understood as before the exam.

Russian past tense verbs agree with gender in the singular.

Here:

• вздохнул = he sighed / took a breath
• вздохнула = she sighed / took a breath

So if the speaker were female, the sentence would be:

Перед экзаменом я глубоко вздохнула и вошла в класс.

Notice both past tense verbs change:

• вздохнул → вздохнула
• вошёл → вошла

Yes, literally it is something like sighed deeply or breathed in deeply, but in natural English this often corresponds to took a deep breath.

Russian commonly uses:

• глубоко вздохнуть = to take a deep breath / to sigh deeply

So this is a normal Russian way to express that idea. It may sound slightly more verbal than the English noun phrase take a deep breath, but the meaning is very natural.

Вздохнул is perfective.

It describes a completed single action: the speaker took one deep breath. In this sentence, the action is seen as a whole, finished event before the next action.

That fits the sequence:

1. вздохнул = took a deep breath
2. вошёл = entered the classroom

If you used the imperfective вздыхал, it would suggest repeated action or a process, such as was sighing or kept sighing, which would not fit this context as well.

Because вошёл means went in / entered, while шёл means was walking / went on foot more generally.

• идти / шёл = to go, to walk
• войти / вошёл = to go in, to enter

Here the important idea is crossing into the classroom, so вошёл в класс means entered the classroom.

Also, вошёл is perfective, which matches the completed action.

Because в can take different cases depending on the meaning:

• в + accusative = motion into somewhere
• в + prepositional = location inside somewhere

So:

• в класс = into the classroom
• в классе = in the classroom

Since the speaker is entering, Russian uses the accusative:

• вошёл в класс = entered the classroom

Russian has no articles like a or the.

So:

• экзамен can mean an exam or the exam
• класс can mean a classroom or the classroom

The exact meaning depends on context. In this sentence, English naturally uses the exam and the classroom, but Russian does not need separate words for that.

Yes, it often could be omitted:

Перед экзаменом глубоко вздохнул и вошёл в класс.

Russian often leaves out subject pronouns when the meaning is clear from context or from the verb form.

However, keeping я is also perfectly normal. It can make the sentence a little clearer or slightly more explicit. In isolated example sentences, learners often see the pronoun included for clarity.

Because Russian, like English, often links sequential actions in one sentence with и when they belong to the same situation.

So:

я глубоко вздохнул и вошёл в класс

means:

• I took a deep breath
• and entered the classroom

This sounds natural because the two actions happen one after the other and involve the same subject.

Yes, Russian word order is fairly flexible.

The neutral order here is:

Перед экзаменом я глубоко вздохнул и вошёл в класс.

But other orders are possible, depending on emphasis. For example:

• Я перед экзаменом глубоко вздохнул и вошёл в класс.
• Глубоко вздохнув, я вошёл в класс перед экзаменом. (more literary / restructured)

The original sentence is natural and straightforward. Putting перед экзаменом first sets the time frame right away, which is very common.

Both can sometimes be translated as went in, but they are not always interchangeable.

• вошёл в класс = entered the classroom
• зашёл в класс = went into the classroom, often with a sense of stepping in, dropping in, or going inside briefly depending on context

In your sentence, вошёл в класс is the most direct and neutral way to say entered the classroom.

Yes, вошёл is spelled with ё, and that shows the stress:

• вошёл = stress on the last syllable

In many Russian texts, ё is often written as е, so you may see вошел, but it is still pronounced вошёл.

For learners, writing ё is very helpful because it makes pronunciation clearer.

Yes. The verb вздохнуть covers both ideas depending on context.

It can mean:

• to sigh
• to take a breath
• sometimes specifically to take a deep breath

With глубоко, the sentence strongly suggests:

• took a deep breath

rather than an emotional sigh, although the two ideas can overlap.

Yes. Both main verbs are perfective past tense, so the sentence presents two finished actions one after another:

1. вздохнул — took a deep breath
2. вошёл — entered the classroom

This is a very common Russian pattern for narrating events. It gives the sentence a clear sense of progression.