النهر طويل.

A common Egyptian pronunciation is en-nahr taweel.

A rough breakdown:

• النهر → en-nahr
• طويل → taweel

Notes:

• The spelling has ال for the, but because ن is a sun letter, the l sound assimilates, so النهر is pronounced en-nahr, not el-nahr.
• ط is a heavier, deeper t sound than English t.
• ee in taweel is a long vowel.

In Arabic, especially in simple present-tense sentences like this, you usually do not say is.

So:

• النهر طويل literally looks like the river long
• but it means The river is long

This is very normal in Arabic. The present-tense to be is usually omitted.

If you wanted past tense, then a form of كان could appear:

• النهر كان طويل = The river was long
  in Egyptian speech

Because Arabic normally puts adjectives after the noun they describe.

So:

• النهر الطويل = the long river

But in your sentence:

• النهر طويل = the river is long

That is slightly different. Here طويل is not just an adjective inside a noun phrase; it is the predicate of the sentence, telling you something about the river.

So an English speaker should notice this important contrast:

• النهر الطويل = the long river
• النهر طويل = the river is long

Because طويل here is a predicate adjective, not part of a noun phrase.

Compare:

1. النهر طويل
   = The river is long
   Here, طويل is the thing being said about the river, so it stays indefinite.

2. النهر الطويل
   = the long river
   Here, طويل directly describes النهر inside one noun phrase, so it must match in definiteness and become الطويل.

This is one of the most important differences for English speakers learning Arabic.

ال is the Arabic definite article, meaning the.

So:

• نهر = a river / river
• النهر = the river

In Egyptian Arabic, ال is often pronounced el- or il-, but with sun letters like ن, the l sound assimilates:

• النهر → en-nahr

So the writing keeps ال, but the pronunciation changes.

A sun letter is a consonant that causes the l in ال to assimilate in pronunciation.

Since ن is a sun letter:

• ال + نهر is pronounced
• en-nahr

The l is not pronounced; instead, the n is doubled in effect.

This matters because learners often expect every written letter to be pronounced separately, but Arabic does not work that way with ال.

نهر is masculine.

That is why the adjective appears in the masculine form:

• طويل = long (masculine)

If the noun were feminine, the adjective would usually also be feminine:

• masculine: طويل
• feminine: طويلة

So if you changed the noun to a feminine one, the adjective would normally change too.

Not very different in writing. The sentence is also correct in Modern Standard Arabic:

• النهر طويل

The biggest differences are usually in:

• pronunciation
• case endings

In Modern Standard Arabic, in fully formal pronunciation, you might hear:

• an-nahru tawīlun

But in Egyptian Arabic, case endings are not used, so you get:

• en-nahr taweel

So for a learner, the written sentence looks almost the same, but the spoken form is simpler in Egyptian.

In Egyptian Arabic, tanween is generally not pronounced in normal speech.

So although in fully formal Modern Standard Arabic you might see or analyze:

• طويلٌ

in Egyptian you simply say:

• طويل → taweel

This is part of a broader pattern:

• Egyptian Arabic drops the case endings
• spoken Arabic is much simpler in this respect than formal written Arabic

A very common Egyptian Arabic way is:

• النهر مش طويل
• en-nahr mish taweel

Here:

• مش = not

So:

• النهر طويل = The river is long
• النهر مش طويل = The river is not long

This is extremely useful because مش is one of the most common negation words in Egyptian Arabic.

Not for the same meaning in normal everyday Arabic.

If you want to say The river is long, the natural structure is:

• النهر طويل

Putting طويل first changes the structure and usually would not be the normal beginner way to say this sentence.

So for now, the safe pattern is:

• [noun] + [adjective/predicate]
• النهر طويل
• الباب مفتوح
• البيت كبير

For A river is long, Arabic usually depends on context, and the sentence without ال can sound more general:

• نهر طويل could mean a long river rather than a full sentence, depending on context.

For The river is long, your sentence is clear:

• النهر طويل

For the long river, you need both words definite:

• النهر الطويل

So the pattern is:

• نهر طويل = a long river
• النهر الطويل = the long river
• النهر طويل = the river is long

Those three can be confusing at first, but they are a key Arabic pattern.

طويل can mean long or tall, depending on what it describes.

Examples:

• النهر طويل = The river is long
• الولد طويل = The boy is tall

So English uses different words depending on the noun, but Arabic often uses طويل for both ideas.

The context tells you whether to understand it as long or tall.

A natural learner-friendly pronunciation is:

• en-NAHR ta-WEEL

The most noticeable stress is usually on:

• nahr
• weel

You do not need to make the stress very dramatic. Just focus on:

• saying en-nahr smoothly
• keeping the ee in taweel long

That will already make your pronunciation much more natural.