لطفا در را ببند؛ صدای خیابان خیلی بلند است.

را marks the direct object. In this sentence, در means the door, and را shows that the door is the thing being acted on.

So:

• در = door
• را = object marker
• ببند = close

Together, در را ببند literally works like the door, close (it).

English does not have a direct equivalent of را, so this often feels strange at first.

ببند is the imperative (command form) of the verb بستن, meaning to close.

The present stem of بستن is بند.
To make a positive command in Persian, you usually add بـ to the stem:

• بند → ببند = close!

So در را ببند means Close the door.

This is a normal singular command, used when speaking to one person informally.

Yes. ببند is the singular imperative used with تو.

If you want to speak more politely or to more than one person, Persian usually uses the plural/formal form:

• در را ببندید = Please close the door.

Your sentence already includes لطفا, which makes it polite, but ببند is still grammatically the informal singular form.

So:

• لطفا در را ببند = polite in tone, but still singular/informal
• لطفا در را ببندید = more standard polite/formal version

Persian normally puts the verb at the end of the clause.

A very common Persian order is:

• subject + object + verb
• or, in commands, often just object + verb

So:

• در را ببند = the door + object marker + close

This is completely normal Persian word order.
English prefers verb + object in commands, but Persian prefers the object before the verb.

لطفا means please.

It softens the command and makes it more courteous:

• در را ببند = Close the door.
• لطفا در را ببند = Please close the door.

A small spelling note: you may also see it written as لطفاً. Both are common, but لطفاً is the more traditional spelling with the tanvīn sign.

صدای خیابان means the sound/noise of the street.

This is an ezafe construction, which links nouns together.
The basic pattern is:

• noun + ezafe + noun

Here:

• صدا = sound, noise
• صدای = sound of ...
• خیابان = street

So:

• صدای خیابان = the sound of the street / street noise

The -e / -ye sound of ezafe is often written only in some cases. Because صدا ends in ا, Persian writes ی to show the ezafe link: صدای.

Because of ezafe.

When a noun is connected to another noun, Persian often adds an ezafe sound, usually pronounced -e or -ye.

Since صدا ends in a vowel sound, the ezafe is pronounced as -ye, and it is written with ی:

• صدا = sound
• صدای = sound of ...

So صدای خیابان literally means sound-of street.

Yes, بلند can mean several related things, including:

• tall
• high
• long
• loud

In this sentence, because it describes sound/noise, بلند means loud:

• صدای خیابان خیلی بلند است = The street noise is very loud.

This is common in Persian. The exact meaning depends on the noun it describes.

Examples:

• دیوار بلند = a tall wall
• صدای بلند = a loud sound/voice

است means is.

So:

• خیلی = very
• بلند = loud
• است = is

Together: is very loud

In formal or careful written Persian, است is often included.
In everyday speech, it is very often shortened or dropped:

• صدای خیابان خیلی بلنده
• صدای خیابان خیلی بلندِ
• sometimes even just صدای خیابان خیلی بلنده

So the written sentence is standard and clear, but in conversation you will often hear a spoken version instead of the full است.

Both are fine.

Literally, it is the sound of the street.
But in natural English, the street noise is often the more idiomatic translation in this context.

Persian صدا can mean:

• sound
• noise
• voice

Here, because the speaker wants the door closed, the intended meaning is clearly noise.

A very common colloquial version would be something like:

• لطفاً درو ببند، صدای خیابون خیلی بلنده.

Changes you may notice:

• در را → درو
• خیابان → خیابون
• است → ه / ـه in speech, giving بلنده

So the written version is more standard:

• لطفا در را ببند؛ صدای خیابان خیلی بلند است.

And the spoken version is more conversational:

• لطفاً درو ببند، صدای خیابون خیلی بلنده.

؛ is a semicolon.

It links two closely related clauses:

• لطفا در را ببند
• صدای خیابان خیلی بلند است

The meaning is basically:

• Please close the door; the street noise is very loud.

In modern Persian, many people would also write this with a comma or a period instead. The semicolon is correct, but it is a bit more formal in appearance.

A few things may help:

• لطفا is pronounced roughly lotf-an or lotfan
• در = dar
• ببند = beband
• صدای = sedâ-ye
• خیابان = khi-yâ-bân
• بلند = boland
• است = ast

In natural speech, some parts are reduced:

• صدای flows together as sedâye
• است is often weakened or replaced by -e

So the full sentence in careful pronunciation is close to:

• lotfan dar râ beband; sedâ-ye khiâbân kheyli boland ast

And in everyday speech it may sound more like:

• lotfan daro beband, sedâye khiâboon kheyli bolande