یکی از صندلی ها نزدیک پنجره است.

یکی از pronounced yeki az, means one of.

It is a very common pattern in Persian:

• یکی از کتاب‌ها = one of the books
• یکی از دوستانم = one of my friends

So here, یکی از صندلی‌ها means one of the chairs.

Because یک and یکی are used a little differently.

• یک usually comes directly before a noun:
  یک صندلی = a chair / one chair
• یکی can stand on its own and means one as a separate item.

In the pattern یکی از ..., Persian uses the standalone form یکی.
So یکی از صندلی‌ها is correct, not یک از صندلی‌ها.

Because the meaning is one out of a group of chairs.

So Persian, like English, uses:

• one of + plural noun

That is why you get:

• یکی از صندلی‌ها not
• یکی از صندلی

The plural noun shows that there are several chairs, and we are selecting one from them.

ها is a very common plural marker in Persian. It makes صندلی into صندلی‌ها = chairs.

About spelling:

• Standard modern spelling usually writes this with a half-space: صندلی‌ها
• Many people type it with a normal space: صندلی ها

Both represent the same thing for a learner, but صندلی‌ها is the more standard written form.

Because the grammatical subject is یکی = one.

Even though chairs is plural, the sentence is really about one member of that group:

• One of the chairs is ... not
• One of the chairs are ...

Persian works the same way here, so است is singular and correct.

Persian does not have a word exactly like the English definite article the.

So nouns often appear without any separate word for the:

• صندلی‌ها can mean chairs or the chairs, depending on context
• پنجره can mean a window or the window, depending on context

In this sentence, English naturally translates it as one of the chairs and the window, even though Persian does not use a separate word for the.

نزدیک means near or close.

Here, نزدیک پنجره means near the window and works as the description/predicate of the chair:

• یکی از صندلی‌ها = the subject
• نزدیک پنجره = the location/description
• است = is

So the structure is basically:

• One of the chairs + near the window + is

That word order is normal in Persian because the verb usually comes at the end.

Yes. In careful pronunciation, many speakers would say an ezafe sound there:

• نزدیکِ پنجره
• pronounced roughly nazdik-e panjere

That -e links نزدیک to پنجره.

In ordinary Persian writing, ezafe is often not written after a word ending in a consonant, so learners have to get used to hearing or supplying it themselves.

Because in this sentence, نزدیک پنجره is not inside the noun phrase the chairs. It is the main statement about one chair.

So:

• یکی از صندلی‌ها نزدیک پنجره است
  = One of the chairs is near the window

But if you said:

• یکی از صندلی‌های نزدیک پنجره ...

that would mean something more like:

• one of the chairs near the window ...

In that version, near the window is part of the noun phrase, not the main predicate.

In everyday speech, the final است is often shortened.

A very natural spoken version is:

• یکی از صندلی‌ها نزدیک پنجره‌ست

A rough transliteration is:

• yeki az sandali-hâ nazdik-e panjere-st

The full form است is perfectly correct, but it sounds a bit more formal or written.