Какую бы книгу ты ни открыла, ты наверняка найдёшь там хотя бы одну полезную мысль.

It is a fixed Russian pattern meaning whatever book you open, whichever book you may open, or no matter what book you open.

Even though it starts with какую, which normally can appear in questions, here it is not asking anything. It is a concessive pattern:

• какую ... ни = whatever / whichever
• бы helps create the non-specific, hypothetical meaning

So the first clause is not a real question. It sets up a general statement:

• Какую бы книгу ты ни открыла, ... = Whatever book you open, ...

Because книгу is the direct object of открыла.

You open a book, so book has to be in the accusative case:

• dictionary form: книга
• accusative singular: книгу

The word какую has to agree with книгу in gender, number, and case:

• какая = nominative feminine singular
• какую = accusative feminine singular

So:

• какую бы книгу = whatever book

Yes, it is the same particle, but here it is not making a normal would-type conditional.

In this sentence, бы is part of the fixed pattern:

• кто бы ни...
• что бы ни...
• где бы ни...
• какую бы ... ни...

This pattern gives a meaning like:

• whoever
• whatever
• wherever
• no matter which

So in какую бы книгу ты ни открыла, бы helps make the book non-specific and hypothetical.

Because ни is part of this special concessive pattern, while не is ordinary negation.

Compare the roles:

• не = not
• ни in this structure = part of the meaning no matter... / whatever...

So ты ни открыла does not mean you did not open. The clause is not negative in the normal sense. Instead, ни works together with какую бы to create the idea:

• no matter which book you open

This is a very common Russian pattern, and it is best learned as a whole.

That is a very common thing in Russian with бы.

Russian often uses a past-tense form together with бы in hypothetical, unreal, or non-specific situations. So here открыла does not literally mean that the opening already happened in the past.

Instead, the clause means something like:

• if you were to open
• whenever you happen to open
• whatever book you may open

So the form looks past, but the meaning is general and hypothetical.

Because the sentence is addressed to a woman or girl.

With ты, the past-tense verb shows gender:

• to a man: ты открыл
• to a woman: ты открыла

So:

• Какую бы книгу ты ни открыл... = said to a male
• Какую бы книгу ты ни открыла... = said to a female

Because найти is a perfective verb, and perfective verbs do not have a true present tense in Russian. Their non-past forms express the future.

So:

• найдёшь = you will find

The perfective aspect fits well here because it focuses on the result: after opening any book, you will find at least one useful thought.

If you used находишь, that would come from the imperfective находить, and it would sound more like a habitual or ongoing process. In this sentence, Russian prefers the result-oriented perfective найдёшь.

Наверняка means something like:

• for sure
• certainly
• almost certainly
• you are bound to

It adds confidence to the statement:

• ты найдёшь = you will find
• ты наверняка найдёшь = you will surely / very likely find

The sentence would still be grammatical without наверняка, but it would sound a little less emphatic.

Хотя бы means at least.

So:

• хотя бы одну полезную мысль = at least one useful thought

It suggests a minimum:

• maybe you will find more than one
• but even in the worst case, you will find one

This is a very common phrase in Russian.

Because this is the direct object of найдёшь, so it is in the accusative case.

Dictionary forms:

• одна
• полезная
• мысль

Accusative forms here:

• одну
• полезную
• мысль

The noun мысль is one of those feminine nouns whose accusative singular looks the same as the nominative singular, but the adjective and numeral still clearly show the accusative:

• одна полезная мысль = nominative
• одну полезную мысль = accusative

Both ideas are possible, but там is very natural in Russian.

Here там means something like:

• there
• in it
• in that book

Russian often treats a book, article, text, or source as a kind of location, so там sounds normal.

You could also say в ней, referring directly to книге, but там is lighter and very idiomatic in this kind of sentence.

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

So you could say:

• Какую бы книгу ни открыла, наверняка найдёшь там хотя бы одну полезную мысль.

That is possible if the context already makes it clear who is being addressed.

Including ты makes the sentence more explicit and conversational.

Because the first part is a subordinate clause and the second part is the main clause.

Structure:

• Какую бы книгу ты ни открыла = subordinate clause
• ты наверняка найдёшь там хотя бы одну полезную мысль = main clause

Russian separates them with a comma, just as English often does in sentences like:

• Whatever book you open, you will find...

The basic meaning would stay the same, but the given word order is very natural.

Starting with:

• Какую бы книгу ты ни открыла, ...

puts emphasis on no matter which book.

Russian word order is flexible, but this pattern is common because it presents the condition first and then the conclusion.

Inside the clause, бы is also very commonly placed right after the question word:

• какую бы книгу...

That placement is standard and natural.