اگر این صندلی خوب نیست، یک صندلی دیگر بیاور.

اگر means if. It introduces the condition:

اگر این صندلی خوب نیست
= if this chair is not good

So the sentence has two parts:

• the if-clause: اگر این صندلی خوب نیست
• the main command: یک صندلی دیگر بیاور

This is a very normal Persian pattern.

In Persian, demonstratives like این (this) and آن (that) normally come before the noun:

• این صندلی = this chair
• آن صندلی = that chair

So this part is very straightforward: این صندلی literally means this chair.

Yes. نیست means is not.

So:

• خوب = good
• نیست = is not

Together:

• خوب نیست = is not good

Persian often says things in the order subject + adjective + is/is not. So literally, this part is closer to:

this chair good is not

That is completely normal Persian sentence structure.

Because خوب here is not being used inside a noun phrase like good chair. It is a predicate adjective, part of the statement the chair is good / is not good.

So:

• این صندلی خوب نیست = this chair is not good

But if you wanted to say a good chair, then you would use ezafe:

• صندلیِ خوب
• یک صندلیِ خوب

So the difference is:

• صندلیِ خوب = a good chair
• صندلی خوب نیست = the chair is not good

It can mean both, depending on context.

Here, یک works like a/an:

• یک صندلی = a chair

In this sentence, because it is followed by دیگر, the whole phrase means:

• یک صندلی دیگر = another chair

So although یک literally means one, it often behaves like an indefinite article in English.

دیگر means other or another.

In Persian, words like this usually come after the noun:

• صندلی دیگر = another chair / other chair

With یک, it very naturally means another chair:

• یک صندلی دیگر = another chair

In careful pronunciation, there is an ezafe-like link in speech, so you may hear something like sandali-ye digar, even though that link is usually not shown in normal writing here.

Because را usually marks a specific direct object.

Here, یک صندلی دیگر is indefinite and non-specific: the speaker is not talking about one particular chair already identified, just another chair in general. So leaving out را is natural.

Compare:

• یک صندلی دیگر بیاور = bring another chair
• صندلی دیگری را بیاور = bring the other chair / bring another specific chair

So the absence of را helps keep the object indefinite.

Because Persian is usually an SOV language: subject–object–verb.

In the command part:

• یک صندلی دیگر = object
• بیاور = verb

So Persian says something like:

another chair bring

That is the normal order.

بیاور is the imperative form of آوردن (to bring).

So:

• آوردن = to bring
• بیاور = bring!

This form is used when speaking to one person in an informal or familiar way.

A more formal or plural version would be:

• بیاورید

In everyday spoken Persian, بیاور is often shortened to بیار.

Because Persian, like English, distinguishes between bring and take.

• آوردن = to bring
• بردن = to take

Here the speaker wants the chair brought to the speaker or to the relevant place, so بیاور is the natural choice.

A common colloquial version would be:

اگه این صندلی خوب نیست، یه صندلی دیگه بیار.

Changes:

• اگر → اگه
• یک → یه
• دیگر → دیگه
• بیاور → بیار

The meaning stays the same, but this version sounds much more conversational.