اگر این کتاب را نمیفهمی، کتاب دیگر را بخوان.

In Persian, adjectives usually come after the noun, not before it.

So:

• کتاب دیگر = another book / other book
• literally: book other

This is normal Persian word order. A native English speaker often wants to say something like دیگر کتاب, but that is not the usual pattern here.

را marks a specific direct object.

In this sentence, both books are the objects of verbs:

• این کتاب را نمی‌فهمی = you do not understand this book
• کتاب دیگر را بخوان = read another/the other book

A useful way to think about را is that it often appears when the object is definite, specific, or clearly identified in context.

Also notice that را comes after the whole noun phrase, not just after the noun:

• این کتاب را
• not این را کتاب

اگر means if.

It introduces a condition:

• اگر این کتاب را نمی‌فهمی = if you don’t understand this book

Then the second part gives the result or advice:

• کتاب دیگر را بخوان = read another book

So the sentence follows the common Persian pattern:

• اگر ... ، ...
• if ... , ...

Persian often does not need a separate word for then, although you can sometimes add one in other contexts.

نمیفهمی is made of several pieces:

• نـ = negation, not
• می = present/imperfective marker
• فهم = verb base related to understanding
• ی = you singular ending

So:

• می‌فهمی = you understand
• نمی‌فهمی = you do not understand

In standard modern writing, this is usually written with a half-space:

• نمی‌فهمی

But many people also write:

• نمیفهمی

Both are commonly seen.

There is actually an ezafe relationship there, even though it is usually not written.

So کتاب دیگر is pronounced roughly like:

• ketâb-e digar

That little -e sound links the noun to what follows it. Persian script often leaves this unwritten unless spelling rules require it.

So even though you see:

• کتاب دیگر

you should understand it as:

• کتابِ دیگر

This is very common in Persian.

Because Persian verbs usually show the subject in their endings, so subject pronouns are often omitted.

In نمی‌فهمی, the ending -ی tells you the subject is you singular (تو).

In بخوان, the command form also shows that the speaker is talking to you singular.

So Persian does not need to say تو here, although it could be added for emphasis:

• اگر تو این کتاب را نمی‌فهمی، کتاب دیگر را بخوان.

Usually, though, leaving it out sounds more natural.

Because بخوان is the imperative form: read!

It comes from خواندن (to read) and is built from the imperative/subjunctive stem:

• بـ
  • خوان = بخوان

So:

• بخوان = read! (to one person, informal)

By contrast:

• می‌خوانی = you read / you are reading

That is not a command; it is a statement.

So in this sentence, the speaker is giving advice or an instruction:

• If you don’t understand this book, read another book.

Yes. بخوان is the singular informal command, used with تو.

If you wanted to speak more formally or politely, you would usually say:

• اگر این کتاب را نمی‌فهمید، کتاب دیگری را بخوانید.

So:

• نمی‌فهمی / بخوان = informal singular
• نمی‌فهمید / بخوانید = formal singular or plural

This is an important distinction in Persian.

Persian often uses دیگر to mean other or another, depending on context.

So:

• کتاب دیگر can mean another book or a different book

You may also hear:

• کتاب دیگری

This is also very common and often feels a bit more explicitly indefinite, like another book.

In this sentence, کتاب دیگر is perfectly natural.

This is a very good question, because English speakers often expect object marking only with something clearly definite like the book.

In Persian, را is usually used with a specific object in the speaker’s mind, and that can include something like کتاب دیگر in context. Here the speaker is not talking about books in general; they mean a specific alternative book: some other book instead of this one.

So کتاب دیگر را بخوان sounds natural because the object is still treated as a concrete, intended object.

The sentence has two parts:

1. اگر این کتاب را نمی‌فهمی

   • if you do not understand this book

2. کتاب دیگر را بخوان

   • read another book

A literal breakdown is:

• اگر = if
• این کتاب را = this book
• نمی‌فهمی = you do not understand
• کتاب دیگر را = another book
• بخوان = read

So the sentence is a very typical Persian conditional plus command pattern:

• If X, do Y.

Yes, it is natural and understandable.

A couple of small notes:

• In careful modern spelling, many writers would prefer نمی‌فهمی with the half-space.
• Some speakers might also say کتاب دیگری را بخوان instead of کتاب دیگر را بخوان.

But the sentence as given is completely normal and idiomatic.