Та книга, о которой ты говорила, лежит на полке рядом с орхидеей.

Та книга means that book and points to a specific book already known from context.

In Russian, тот / та / то / те works like that:

• тот стол = that table
• та книга = that book
• те люди = those people

Here, та книга helps identify which book is meant, and the relative clause о которой ты говорила gives extra information about it: that book, the one you were talking about.

If you said only книга, it would just mean the book / a book, depending on context, and would sound less specific.

Because книга is a feminine noun.

The demonstrative has to agree with the noun in:

• gender
• number
• case

So in the nominative singular:

• masculine: тот
• feminine: та
• neuter: то

Since книга is feminine singular nominative, you get та книга.

Эта книга would mean this book, not that book.

Because книга is the subject of the main clause.

The main structure is:

• Та книга ... лежит на полке
  = That book ... is lying on the shelf

The thing doing the main action/state is книга, so it stays in the nominative.

The relative clause in the middle—о которой ты говорила—just adds information about the book. It does not change the case of книга in the main clause.

О которой means about which or, more naturally in English, that ... about in the phrase the book that you were talking about.

It is made of:

• о = about
• которой = a form of которая / который meaning which / that

So:

• о которой ты говорила = about which you were speaking
• natural English: that you were talking about

Russian often uses forms of который in relative clauses where English uses who, which, that, or even no relative word at all.

Because the pronoun must match the grammar required inside the relative clause.

The verb is говорить о чём-то = to talk about something.
After о, Russian uses the prepositional case.

So the relative pronoun referring back to книга must be:

• feminine singular
• prepositional

That form is которой.

A quick comparison:

• nominative feminine: которая
• prepositional feminine: о которой
• accusative feminine: которую

Since the phrase is говорить о книге, the relative version becomes книга, о которой...

In the past tense, Russian verbs agree with the subject in gender and number.

So:

• ты говорил = you were speaking / you talked (to a male)
• ты говорила = you were speaking / you talked (to a female)

Here, говорила tells you that the person being addressed by ты is female.

This is something English does not show in the verb, but Russian does in the past tense.

Yes, very often.

Russian often drops subject pronouns when the meaning is clear from the verb form or context.

So both are possible:

• Та книга, о которой ты говорила, лежит на полке...
• Та книга, о которой говорила, лежит на полке...

However, keeping ты can:

• make the sentence clearer
• add emphasis
• sound more natural in some contexts

Because говорила already shows feminine singular, the listener can often infer you from context.

Because it is a relative clause inserted into the main sentence.

The full sentence has:

• main clause: Та книга лежит на полке рядом с орхидеей.
• inserted relative clause: о которой ты говорила

Russian normally sets off this kind of descriptive relative clause with commas:

• Та книга, о которой ты говорила, ...

This is similar to English punctuation in sentences like:

• That book, which you were talking about, is on the shelf...

Even though English sometimes prefers that without commas, Russian commonly uses commas with such subordinate clauses.

Russian usually does not use a present-tense form of to be in sentences like this.

So instead of saying something like The book is on the shelf with a present-tense is, Russian often uses:

• no verb at all in some sentences, or
• a verb describing position/state, such as лежит, стоит, висит

Here, лежит literally means lies / is lying, and it is very natural for objects resting horizontally, like a book on a shelf.

Compare:

• Книга на полке. = The book is on the shelf.
• Книга лежит на полке. = The book is lying on the shelf.

The second version is a bit more vivid and specific.

На полке is prepositional case.

The preposition на can take different cases depending on meaning:

• на + accusative = motion onto something
• на + prepositional = location on something

Compare:

• положить книгу на полку = to put the book onto the shelf
• книга лежит на полке = the book is on the shelf

Since this sentence describes location, not movement, Russian uses на полке.

After рядом с (next to / beside), Russian uses the instrumental case.

So:

• рядом с чем? = next to what?
• рядом с орхидеей = next to an orchid

Орхидея is a feminine noun ending in -я. In the instrumental singular, it becomes орхидеей.

So the pattern is:

• nominative: орхидея
• instrumental: с орхидеей

This is the same case used after the preposition с in many expressions of accompaniment or position.

Because that is the normal instrumental singular ending for many feminine nouns ending in -ия / -ея / -я.

For орхидея:

• base form: орхидея
• instrumental singular: орхидеей

Some similar examples:

• линия → линией
• идея → идеей
• Мария → Марией

So in рядом с орхидеей, the noun changes because of the grammar required by с after рядом.

Yes. Russian word order is flexible, though the default order here is very natural.

This sentence:

• Та книга, о которой ты говорила, лежит на полке рядом с орхидеей.

could be rearranged, for example:

• На полке рядом с орхидеей лежит та книга, о которой ты говорила.

Both are grammatical, but the emphasis changes:

• starting with Та книга focuses first on the book
• starting with На полке... focuses first on the location

Russian often uses word order to manage emphasis and information flow, not just grammar.

No, not in this sentence.

Которую would be accusative feminine singular, and it would only work if the verb required a direct object.

But говорить in the meaning talk about requires о + prepositional, not a direct object:

• говорить о книге = talk about a book

So the correct relative phrase is:

• книга, о которой ты говорила

If you used которую, it would suggest a different grammatical relationship and would be wrong here.

Both can work depending on context.

Russian past tense by itself does not always force the same distinction English makes between:

• spoke
• was talking
• have talked

So о которой ты говорила can mean:

• that you spoke about
• that you were talking about
• sometimes simply that you mentioned

The best English choice depends on the surrounding situation, not just on the Russian verb form alone.