Сегодня первая пара по истории начинается в восемь часов.

In this sentence, пара does not mean a literal pair of objects. In school and university contexts, пара is a common word for one class block, especially at a university. Traditionally, it means a double period: two standard lesson units treated as one block.

So первая пара means the first class period / the first class block, not the first pair in the everyday English sense.

In some contexts, a learner might also hear урок for lesson, but пара is especially common in higher education schedules.

Because пара is a feminine noun in Russian.

Adjectives and ordinal numbers must agree with the noun they describe in:

• gender
• number
• case

Since пара is feminine singular, first becomes первая.

So:

• первый = masculine
• первая = feminine
• первое = neuter

Here:

• первая пара = the first class

По истории means for history / in history / history-related, and here the most natural English translation is simply history or history class.

The preposition по often takes the dative case in this kind of meaning, especially when talking about a subject or field.

The noun история changes like this:

• nominative: история
• dative: истории

So:

• по истории = for history / in history / on the subject of history

This is a very common pattern:

• пара по математике = math class
• лекция по биологии = lecture on biology
• экзамен по русскому языку = Russian exam

Because Russian normally uses the reflexive verb начинаться when something begins by itself.

So:

• начинать = to begin/start something
  
  • for example, Он начинает урок = He starts the lesson
• начинаться = to begin/start
  
  • for example, Урок начинается = The lesson begins

In your sentence, the subject is первая пара. The class is not starting something else; the class itself is beginning. So Russian uses:

• первая пара начинается = the first class begins

That is why -ся appears here.

The ending -ся often marks a verb as reflexive, but in practice it does several jobs in Russian. It does not always mean a literal English reflexive like oneself.

Here, in начинается, it helps form the intransitive verb начинаться, meaning to begin.

So:

• начинать = to start something
• начинаться = to begin

In this sentence, -ся is part of the normal dictionary form of the verb you need.

Russian word order is more flexible than English word order. Putting Сегодня first sets the time frame right away: Today...

The sentence could be rearranged, for example:

• Сегодня первая пара по истории начинается в восемь часов.
• Первая пара по истории сегодня начинается в восемь часов.
• В восемь часов сегодня начинается первая пара по истории.

All of these are possible, but they sound slightly different in emphasis.

With Сегодня first, the speaker is naturally framing the whole statement around today. That is a very normal choice.

Первая пара is in the nominative case, because it is the subject of the sentence.

It is the thing that begins:

• первая пара begins

So:

• первая = nominative feminine singular
• пара = nominative feminine singular

When Russian gives a specific clock time at which something happens, it usually uses:

• в
  • accusative

So:

• в один час = at one o’clock
• в три часа = at three o’clock
• в восемь часов = at eight o’clock

Here, восемь is in the accusative, but for this number it looks the same as the nominative.

The noun час changes because of the number:

• 1 → час
• 2, 3, 4 → часа
• 5 and higher → часов

That is why we get:

• в восемь часов

Because Russian numbers affect the form of the noun that follows them.

With time expressions:

• один час = one o’clock
• два часа = two o’clock
• три часа = three o’clock
• четыре часа = four o’clock
• пять часов = five o’clock
• восемь часов = eight o’clock

After 5 and above, Russian uses the genitive plural, so час becomes часов.

That is why the sentence says:

• в восемь часов

Yes, very often.

Both are possible:

• в восемь
• в восемь часов

The version with часов is a bit fuller and more explicit. In everyday speech, people very often shorten it to just в восемь, especially when it is obvious they are talking about time.

So the sentence could also be:

• Сегодня первая пара по истории начинается в восемь.

Russian has no articles like English a and the.

So Russian simply says:

• первая пара

and the meaning is understood from context:

• the first class
• or sometimes a first class period, depending on the situation

In this sentence, English naturally uses the:

• Today the first history class starts at eight o’clock.

But Russian does not need a separate word for it.

In a literal sense, по истории can feel like on the subject of history. But in this sentence, the natural English meaning is just history or history class.

So:

• первая пара по истории = the first history class

A very word-for-word translation like the first class about history would sound awkward in English, even though it helps show how Russian is built.

So for understanding grammar:

• по
  • dative = related to the subject area

For natural translation:

• history class

No, not in normal context.

Because пара in educational Russian is a set phrase for a class block, and по истории clearly identifies the subject. So a Russian speaker will understand this as a class schedule sentence, not as something about a literal pair.

The intended meaning is:

• Today the first history class starts at eight o’clock.

Начинается is imperfective.

The verb pair is:

• начинаться = imperfective
• начаться = perfective

In this sentence, the imperfective is natural because it states a regular fact or scheduled event:

• The first history class starts at eight today.

Russian often uses the imperfective for timetables, routines, and general statements about when something starts.

A perfective form like начнётся would usually emphasize the single future starting point:

• Первая пара начнётся в восемь часов. = The first class will start at eight o’clock.

That is also possible, but it has a slightly different feel.

Because сегодня is an adverb meaning today.

English also uses today without a preposition:

• Today the class starts at eight.

So Russian does the same:

• Сегодня первая пара...

It is not a noun phrase here; it is simply a time adverb modifying the whole sentence.