Если завтра эта акция закончится, кефир снова будет дороже.

In Russian, after если (if), you normally use the same tense that the action really belongs to. Since the sale ending is in the future, Russian uses a future form:

• Если акция закончится... = If the sale ends / if the sale is over...

This is different from English, where future meaning after if is often expressed with the present tense.

Here, закончится is the future tense of the perfective verb закончиться. It means the action will be completed.

So:

• Если акция закончится, ... = natural Russian
• Если акция закончится завтра, ... = If the sale ends tomorrow, ...

Russian does not avoid the future here the way English does.

Russian uses the perfective aspect here because the sentence refers to a completed event: the sale will come to an end.

• заканчиваться = imperfective, focusing on process or repetition
• закончиться = perfective, focusing on completion

In this sentence, the important idea is not that the sale is in the process of ending, but that it will be over. That is why закончится is used.

Compare:

• Если акция закончится... = If the sale ends / is over...
• Акция заканчивается. = The sale is ending / The sale is coming to an end

Yes, акция has several meanings in Russian, and context decides which one is meant.

Common meanings include:

• акция = a sale, special offer, promotion
• акция = a public action, campaign, event
• акция = a stock/share in a company

In the sentence эта акция закончится, кефир снова будет дороже, the mention of кефир becoming more expensive again clearly shows that акция means a store promotion / special offer / discount campaign.

So here:

• эта акция = this promotion / this sale

Because эта акция is the subject of the verb закончится.

The verb answers the question what will end?
Answer: эта акция

Since it is the subject, it must be in the nominative case:

• эта = feminine nominative singular
• акция = feminine nominative singular

If it were the direct object, then you might see эту акцию, but that is not the role here.

In Russian, завтра by itself means tomorrow, and it does not need a preposition.

So:

• завтра = tomorrow
• если завтра эта акция закончится... = if this sale ends tomorrow...

Russian often uses simple time adverbs without prepositions:

• сегодня = today
• вчера = yesterday
• завтра = tomorrow

A preposition is only used in other kinds of time expressions, such as:

• в понедельник = on Monday
• через неделю = in a week

Because the sentence is talking about a future state: kefir will be more expensive again.

Russian often omits to be in the present tense, but not in the future.

So:

• Кефир дороже. = Kefir is more expensive.
• Кефир будет дороже. = Kefir will be more expensive.

Since the sale has not ended yet, the higher price belongs to the future, so будет is necessary.

Дороже is the normal comparative form of дорогой (expensive).

• дорогой = expensive
• дороже = more expensive

Russian usually prefers this short comparative form in sentences like this:

• Кефир будет дороже. = Kefir will be more expensive.

You can sometimes use более дорогой, but it is less natural here and usually sounds more formal or heavy.

Also, дороже does not change for gender, number, or case in this use. It stays the same:

• молоко дороже
• машина дороже
• товары дороже

Russian does not always have to state the thing being compared. It can be understood from context.

Here, дороже means:

• more expensive than it is during the promotion
• more expensive than now

So the comparison is implied, and чем is not needed.

If you wanted to state the comparison explicitly, you could say:

• Кефир будет дороже, чем сейчас. = Kefir will be more expensive than now.
• Кефир будет дороже, чем по акции. = Kefir will be more expensive than at the sale price.

But in the original sentence, that information is already obvious.

Снова means again.

In this sentence, it means that kefir used to be more expensive, then became cheaper because of the promotion, and after the promotion ends it will again be more expensive.

• кефир снова будет дороже = kefir will be more expensive again

Its position is fairly flexible, though different placements can slightly change emphasis:

• кефир снова будет дороже = neutral
• снова кефир будет дороже = stronger emphasis on again
• кефир будет снова дороже = also possible

The original version is natural and neutral.

Because кефир is the subject of the main clause:

• кефир ... будет дороже = kefir ... will be more expensive

The verb phrase будет дороже describes кефир, so кефир stays in the nominative case.

This is similar to:

• Молоко будет холоднее. = The milk will be colder.
• Бензин будет дешевле. = Gasoline will be cheaper.

Yes. Russian word order is more flexible than English word order, because case endings show grammatical roles.

The original sentence is neutral and natural:

• Если завтра эта акция закончится, кефир снова будет дороже.

But other versions are also possible, with different emphasis:

• Если эта акция завтра закончится, кефир снова будет дороже.
• Кефир снова будет дороже, если завтра эта акция закончится.

These all mean basically the same thing, but the original flows very naturally: time first, then subject, then verb.

The verb закончиться means to end, to come to an end by itself.

This reflexive form is commonly used for events, periods, and situations:

• урок закончится = the lesson will end
• фильм закончится = the film will end
• акция закончится = the sale will end

The non-reflexive закончить usually means to finish something:

• Я закончил работу. = I finished the work.

So:

• акция закончится = the sale itself will end
• магазин закончит акцию = the store will end the promotion

Both are possible in principle, but the reflexive version is more natural when the event itself is presented as coming to an end.