Learn Farsi: خیابان‌های شلوغ کثیف نیستند.

This is the Ezafe particle used to connect a noun to its adjective. Here, it connects خیابان‌ها (streets) to شلوغ (crowded). Because the plural suffix ها (ha) ends in a vowel sound (a), the Ezafe is pronounced and written as -ye instead of the usual -e to make it easier to say.

Why isn't there an Ezafe between شلوغ (crowded) and کثیف (dirty)?

The Ezafe is only used to link words within a single phrase, like a noun and its describing adjective (the crowded streets). کثیف (dirty) is part of the verb phrase alongside نیستند (are not). Since it is stating what the streets are not, rather than attaching directly to the noun phrase, no Ezafe is needed here.

How do we make the word for street plural in this sentence?

We add the standard plural suffix ها (-ha) to the singular noun خیابان (khiyaban) to get خیابان‌ها (khiyaban-ha). The Ezafe is then attached to the end of this plural suffix, which is why the full word looks so long.

Why does the negative verb نیستند end in -ند (-nd)?

The suffix -ند is the verb ending for the third-person plural (they). It is attached to نیست (is not). In standard written Farsi, plural inanimate subjects like streets take a plural verb to match, though in casual spoken Farsi you might often hear a singular verb used for things that are not human.