الشقة الجديدة هادئة، لكن الشارع ليس هادئا.

In Arabic, the present-tense verb to be is usually left out in simple statements.

So:

• الشقة الجديدة هادئة literally looks like the apartment the-new quiet
• but it means The new apartment is quiet

This kind of sentence is often called a nominal sentence.

If you want past or future, Arabic does use a verb, for example forms of كان. For present-time negation, Arabic commonly uses ليس, which is what you see in the second clause.

Because Arabic adjectives normally come after the noun they describe.

So:

• الشقة الجديدة = the new apartment
• literally: the apartment the-new

This is the normal Arabic order: noun + adjective.

Because adjectives in Arabic agree with the noun in definiteness as well as gender and number.

So:

• الشقة الجديدة = the new apartment
• شقة جديدة = a new apartment

If the noun is definite, the adjective must also be definite.

Because شقة is a feminine noun, and Arabic adjectives agree with the noun in gender.

So:

• masculine: هادئ = quiet
• feminine: هادئة = quiet

That is why you get:

• الشقة الجديدة هادئة for apartment (feminine)
• الشارع ليس هادئا for street (masculine)

A good clue is that many feminine nouns end in ـة, as شقة does.

ليس is a standard way to negate a present-tense nominal sentence in Modern Standard Arabic.

Compare:

• الشارع هادئ = The street is quiet
• الشارع ليس هادئًا = The street is not quiet

So ليس is doing the job of is not here.

There are two things happening:

1. Masculine vs. feminine

   • first clause: هادئة because it refers to الشقة
   • second clause: هادئ because it refers to الشارع

2. Case ending after ليس

   • in careful MSA, the adjective after ليس takes the accusative ending
   • so the fully vocalized form is هادئًا

In normal Arabic writing, short vowels are often omitted, but the accusative spelling with the extra written ا often remains. That is why you see هادئا.

It does if the text is fully vowelled:

• هادئةٌ in the first clause
• هادئًا in the second clause

But everyday Arabic writing usually leaves off short-vowel marks. So the ending on هادئةٌ is not visible unless diacritics are added.

Also, feminine words ending in ـة do not take the extra written ا that many masculine indefinite accusative words do. So the difference is less obvious in normal spelling.

Although the definite article is written الـ, it is not pronounced al- before every letter.

Here, both words begin with ش, which is a sun letter. The l sound of الـ assimilates to the next consonant.

So:

• الشقة is pronounced roughly ash-shaqqa
• الشارع is pronounced roughly ash-shāriʿ

The spelling stays the same, but the pronunciation changes.

لكن means but or however.

It connects the two clauses:

• الشقة الجديدة هادئة
• لكن الشارع ليس هادئا

So the sentence is making a contrast: the apartment is quiet, but the street is not.

Not exactly.

• لكن is the simple conjunction but/however
• لكنّ is a different grammatical particle that is followed by a noun and affects its case

In this sentence, لكن is the straightforward choice because it simply links two full clauses.

A learner will often first meet لكن as the basic word for but.

Yes. That is also correct.

Arabic word order is somewhat flexible, so both of these are possible:

• الشارع ليس هادئًا
• ليس الشارع هادئًا

They mean the same thing: The street is not quiet.

The version with الشارع first feels more like starting with the topic the street. The version beginning with ليس can sound a bit more formal or more directly negative.

Yes. A common alternative is:

• الشارع غير هادئ

This also means The street is not quiet.

A useful difference is:

• ليس هادئًا = direct sentence negation
• غير هادئ = not quiet / unquiet as an adjectival idea

Both are good Arabic, but ليس هادئًا is especially common in textbook explanations of nominal-sentence negation.