Если тебе станет грустно, непременно прочитай ту главу, где героиня наконец улыбается.

Because Russian uses the dative for the person who experiences a feeling in sentences like this.

• тебе станет грустно = literally something like to you it will become sad
• This is a very common Russian pattern with words like грустно, холодно, весело, трудно, etc.

So:

• ты = subject form, you
• тебе = dative form, to you / for you

Russian is not saying you will become sad in the same grammatical way English does. It is using an impersonal structure.

Станет comes from стать, which means to become. So станет грустно means you become sad / you start feeling sad.

That is slightly different from будет грустно, which means it will be sad or you will feel sad more as a state, without focusing on the change.

Compare:

• Если тебе станет грустно... = If you start to feel sad...
• Если тебе будет грустно... = If you are feeling sad...

Both are possible in many situations, but станет грустно emphasizes the moment the sadness comes on.

Here грустно is not functioning like a normal adjective modifying a noun. It is a predicative word expressing a state or feeling.

In English we often translate it with an adjective:

• Мне грустно = I am sad
• Ему скучно = He is bored
• Нам весело = We are having fun

But grammatically, Russian is using a different structure. There is no ordinary subject like я or он here. Instead, the experiencer goes in the dative:

• мне
• тебе
• ему
• нам

This is about aspect.

• прочитай is perfective
• читай is imperfective

Here прочитай means read it through / read this specific chapter as a complete action, one time.

That fits the context well: someone is recommending a particular action to do when you feel sad.

Compare:

• Прочитай ту главу = Read that chapter
• Читай ту главу can sound more like Read that chapter (as an ongoing activity) or Keep reading that chapter, depending on context

When Russian gives a one-time instruction with a result in mind, the perfective imperative is very common.

Непременно means something like:

• definitely
• certainly
• by all means
• be sure to

It adds a sense of strong encouragement or insistence.

So:

• непременно прочитай = definitely read / be sure to read

It is a bit stronger and slightly more literary or emphatic than simply leaving it out.

Because глава is the direct object of прочитай, so it must be in the accusative case.

Since глава is feminine singular:

• nominative: та глава
• accusative: ту главу

Both words change:

• та → ту
• глава → главу

So прочитай ту главу means read that chapter.

Ту usually points to something more removed, less immediate, or already identified as that one rather than this one.

So:

• эту главу = this chapter
• ту главу = that chapter

In context, ту главу suggests a specific chapter the speaker has in mind and expects the listener to identify, perhaps one already mentioned or known from the book.

This is a very natural Russian usage.

Russian often uses где not only for real places, but also for parts of texts, stories, films, and similar things, meaning the place in the text where...

So:

• та глава, где героиня наконец улыбается = that chapter where the heroine finally smiles

In more formal or literal Russian, you could also say:

• та глава, в которой героиня наконец улыбается

Both are correct. Где sounds very natural and conversational here.

Russian often uses the present tense when talking about what happens in a book, film, or story. English does this too sometimes:

• This is the chapter where the heroine finally smiles
• In this scene the hero dies
• At the end she realizes the truth

So улыбается is a kind of narrative/literary present. It refers to the content of the chapter, not to something happening right now in real life.

It sounds natural because the speaker is identifying the chapter by what happens in it.

Yes, but it would change the feel.

• где героиня наконец улыбается sounds like the chapter where the heroine finally smiles, using plot-summary style
• где героиня наконец улыбнулась sounds more like pointing to the chapter in which that completed event happened

The present tense is especially common when summarizing stories or identifying scenes and chapters. That is why улыбается feels very natural here.

Russian puts commas around subordinate clauses very consistently.

There are two subordinate clauses here:

1. Если тебе станет грустно
   This is an if-clause, so it is separated by a comma from the main clause.

2. где героиня наконец улыбается
   This is a relative clause describing ту главу, so it is also separated by a comma.

Structure:

• Если тебе станет грустно, main clause
• непременно прочитай ту главу, noun being described
• где героиня наконец улыбается. relative clause

Yes, Russian word order is flexible, but the original order sounds natural and neutral.

The sentence starts with the condition:

• Если тебе станет грустно...

Then gives the main advice:

• непременно прочитай ту главу...

That order is very logical: condition first, recommendation second.

You could move things around for emphasis, but the original version is the most straightforward and idiomatic for everyday Russian.

Наконец means finally / at last.

It suggests that the heroine has not smiled for a long time, or that the smile happens after some emotional buildup.

So:

• героиня наконец улыбается = the heroine finally smiles

It adds an emotional sense of relief or completion.