Learn Farsi: مقدار آب ناکافی است و آن‌ها نگران هستند. - The amount of water is insufficient and they are worried.

The prefix nâ- works a lot like "in-" or "un-" in English. It attaches directly to the adjective kâfi (enough or sufficient) to create a brand new negative adjective, nâkâfi (insufficient).

Could I just say meqdâr-e âb kâfi nist instead of using nâkâfi ast?

Yes, you can! kâfi nist means "is not enough" (negating the verb), while nâkâfi ast means "is insufficient" (using a negative adjective). Using nâ- adjectives like nâkâfi elevates your vocabulary and makes your Farsi sound more formal and precise.

Why is there no ezâfe linking âb and nâkâfi?

Because nâkâfi is not acting as an attributive adjective attached to the noun (which would mean "the insufficient water"). Instead, it is the predicate of the sentence ("the amount of water is insufficient"). The ezâfe only appears in meqdâr-e âb (amount of water) to link those two nouns together.

Why do we use ast in the first half but hastand in the second half?