Оглавление было коротким, зато каждая глава обещала интересную тайну.

This is a very common question.

In Russian, after быть in the past or future, a predicate adjective can often appear in:

• nominative: Оглавление было короткое
• instrumental: Оглавление было коротким

Both are possible, but they are not always felt exactly the same.

Here, коротким is the instrumental singular neuter form of короткий. Instrumental is very common in predicate phrases after быть, especially in more neutral or literary-style statements.

Very roughly:

• было короткое = a bit more direct, descriptive
• было коротким = very standard predicate usage, often slightly more formal/bookish

So the sentence is completely normal with коротким.

Yes, it could.

Оглавление было короткое, зато каждая глава обещала интересную тайну.

That version is also grammatical. The difference is mostly stylistic, not a major change in meaning.

For learners, it is useful to remember this:

• after быть, both nominative and instrumental can appear with adjectives
• the instrumental version is especially common in more polished written Russian

So in this sentence, коротким is not “more correct” than короткое, but it is a very natural choice.

Зато means something like:

• but on the other hand
• but in compensation
• but at least
• however, the positive side is that...

So the idea is:

• the table of contents was short
• but this is balanced by something good: each chapter promised an interesting mystery

This is different from plain но, which is just but.

Compare:

• Оглавление было коротким, но каждая глава обещала интересную тайну.
  = simple contrast

• Оглавление было коротким, зато каждая глава обещала интересную тайну.
  = the second part makes up for the first part

So зато gives a stronger feeling of compensation.

Because Russian usually expresses each/every + singular noun with:

• каждый
  • singular noun

So:

• каждая глава = each chapter / every chapter

This is exactly parallel to English every chapter, which also uses a singular noun.

Here both words are feminine singular nominative because глава is feminine and is the subject of обещала:

• каждая = feminine singular nominative
• глава = feminine singular nominative

Because it agrees with the subject глава.

In the past tense, Russian verbs change by gender and number:

• обещал = masculine
• обещала = feminine
• обещало = neuter
• обещали = plural

Since глава is a feminine singular noun, the verb must be обещала.

So:

• каждая глава обещала = each chapter promised

Yes. This is natural in Russian, just as in English you can say:

• The book promises surprises
• The title promises adventure
• The first chapter promises a lot

So обещала here is not literal speech. It means something like:

• suggested
• gave reason to expect
• hinted at

In other words, each chapter looked as if it would contain an interesting mystery.

Because it is the direct object of обещала.

The verb обещать normally takes what is being promised in the accusative:

• обещать помощь = to promise help
• обещать сюрприз = to promise a surprise
• обещать тайну = to promise a mystery

Since тайна is a feminine noun, its accusative singular form is тайну.

The adjective must match it:

• nominative: интересная тайна
• accusative: интересную тайну

So:

• обещала интересную тайну = promised an interesting mystery

Because adjectives must agree with the noun they describe in:

• gender
• number
• case

The basic form is:

• интересная тайна = nominative singular feminine

But in this sentence, тайну is accusative singular feminine, so the adjective also changes:

• интересную тайну

This is standard adjective agreement.

They are close, but not always identical.

• оглавление = table of contents, literally a list of chapter headings/titles
• содержание can also mean table of contents, but it is a broader word and can also mean content in general

In many real contexts, Russians use both for the contents page of a book, but:

• оглавление often emphasizes the list of headings/chapters
• содержание can be more general

In this sentence, оглавление is a very natural choice because the next clause talks about каждая глава.

Yes, Russian word order is flexible, and it can be changed for emphasis.

The version in the sentence is the most neutral and natural:

• Оглавление было коротким, зато каждая глава обещала интересную тайну.

You could also say things like:

• Оглавление было коротким, зато интересную тайну обещала каждая глава.

That version puts more emphasis on интересную тайну.

Russian often uses word order to manage emphasis and flow, not just grammar. The grammatical relationships are still clear because the endings show the roles of the words.

Because this sentence is not about a single completed act of “making a promise.” It is about a descriptive, ongoing impression.

• обещала (imperfective) = was promising / suggested / seemed to promise
• пообещала (perfective) = made a promise once, completed the act

Here the meaning is that each chapter gave the impression of containing an interesting mystery. That fits обещала much better.

Using пообещала would sound much more like a one-time completed event, which is not the intended idea here.

It can mean both, depending on context.

• тайна = secret
• тайна = mystery

In this sentence, интересную тайну most likely means something like:

• an intriguing mystery
• an interesting secret to uncover

Because the context is literary and talks about chapters, the word suggests something hidden, suspenseful, or mysterious.

So even if English might choose mystery in translation, the Russian word still keeps some of the flavor of secret.

It has two clauses joined by зато:

1. Оглавление было коротким
   = The table of contents was short

2. каждая глава обещала интересную тайну
   = each chapter promised an interesting mystery

So the full logic is:

• one thing may seem disappointing: the table of contents was short
• but this is compensated by something positive: every chapter looked intriguing

This kind of contrast is exactly why зато works so well here.