Hiki ni kitabu kilekile nilichokisoma jana.

Because hiki can stand on its own and mean this one.

So the structure is:

• Hiki = This one / This
• ni kitabu... = is a book / is the book...

That makes the whole beginning Hiki ni... = This is...

If you said kitabu hiki, that would mean this book as a noun phrase, not this is a book as a full statement.

Because kitabu belongs to noun class 7 in the singular, and words that refer to it must agree with that class.

For kitabu:

• singular: kitabu
• plural: vitabu

Class 7/8 agreement gives you forms like:

• hiki = this one, for singular kitabu
• kile = that one, for singular kitabu
• kicho / cho / ki- type markers also show up in the verb

An English speaker often expects words like this and that not to change, but in Swahili they do change to match the noun class.

Ni is the copula here, meaning is.

So:

• Hiki ni kitabu = This is a book / This is a book

In Swahili, ni is commonly used in sentences that identify one thing as another:

• Mimi ni mwalimu = I am a teacher
• Huyu ni rafiki yangu = This is my friend

So in your sentence, ni simply links hiki and kitabu kilekile...

Kilekile means the same or that same one.

It comes from repeating kile:

• kile = that for class 7 singular
• kilekile = that very same / the same

This kind of repetition is a common way in Swahili to add emphasis or the idea of exact sameness.

So:

• kitabu kilekile = the same book

In natural English, you usually translate it just as the same book, not literally as that-that book.

It may look strange from an English point of view, but it is completely normal.

The two words are doing different jobs:

• hiki points to the thing being shown now: this one
• kilekile identifies it as the same one mentioned or remembered from before: the same book

So the meaning is:

• This is the same book...

In kilekile, the original that idea is weaker than in plain kile. The whole form works as a fixed expression meaning the same.

A useful breakdown is:

• ni- = I
• -li- = past tense
• -cho- = relative marker agreeing with kitabu
• -ki- = object marker for kitabu
• soma = read

So the whole form means something like:

• that I read it
• more natural English: that I read or which I read

This is a very Swahili way of packing a lot of grammar into one verb.

-cho- is the relative marker. It connects the verb to the noun being described.

Here, the noun being described is kitabu, so the clause means:

• kitabu ... nilichokisoma
• the book that I read

Because kitabu is class 7 singular, the relative marker appears as -cho-.

So -cho- is the part that gives the meaning of that / which in the relative clause.

Because the two pieces do different jobs:

• -cho- = marks the clause as relative and links it to kitabu
• -ki- = marks kitabu again as the object of read

So Swahili is being very explicit:

• the book
• that
• I read it

In English, we usually do not say the book that I read it. We just say the book that I read. But Swahili can keep that object marking inside the verb.

So although it feels redundant to an English speaker, it is a normal Swahili pattern.

Because Swahili does not have articles like a, an, and the.

Whether kitabu means a book or the book depends on context.

In this sentence, it is clearly definite because of the surrounding words:

• hiki = this
• kilekile = the same

So kitabu is understood as the book, not just any book.

Jana means yesterday, and putting it at the end is a very natural Swahili word order.

So:

• nilichokisoma jana = that I read yesterday

Swahili often places time expressions after the verb or after the whole verb phrase. English can do something similar:

• the book that I read yesterday

So the placement of jana here is straightforward and natural.

Yes. A rough guide is:

• Hiki = this one
• ni = is
• kitabu = book
• kilekile = the same
• nilichokisoma = that I read it / which I read
• jana = yesterday

So a rough literal sense is:

• This is book the-same that I read it yesterday

The natural English meaning is:

• This is the same book that I read yesterday.

That difference between the rough literal sense and the natural translation is very normal when learning Swahili grammar.