الكم واسع.

It is a nominal sentence in Arabic, meaning a sentence built without a finite verb in the present tense.

Its two main parts are:

• الكم — the subject of the sentence
• واسع — the predicate describing the subject

In traditional Arabic grammar, these are called:

• مبتدأ for the subject
• خبر for the predicate

So the structure is simply:

• the sleeve
  • wide

which gives the meaning already shown to the learner.

In Arabic, the verb to be is usually not expressed in the present tense.

So instead of saying:

• the sleeve is wide

Arabic simply says:

• the sleeve wide

This is completely normal in Modern Standard Arabic.

If you wanted a past or future meaning, Arabic would usually use a verb, for example:

• كان الكم واسعًا — the sleeve was wide

So in the present tense, leaving out is is standard.

Because الكم is the definite subject, while واسع is the predicate.

In Arabic nominal sentences, the predicate is often indefinite, even when the subject is definite.

So:

• الكم واسع = the sleeve is wide

But if you said:

• الكم الواسع

that would usually no longer be a full sentence. It would mean:

• the wide sleeve

So this is an important distinction:

• الكم واسع = a sentence
• الكم الواسع = a noun phrase

Yes, it agrees in gender and number.

Here, الكم is treated as:

• masculine
• singular

So the predicate is also masculine singular:

• واسع

However, in this kind of sentence, the predicate does not have to match the noun in definiteness. That is why it is واسع, not الواسع.

So the agreement works like this:

• gender — yes
• number — yes
• definiteness — not necessarily, because this is a predicate, not an attributive adjective

With full case endings, it would normally be written:

• الكُمُّ واسِعٌ

This shows that both words are in the nominative, because:

• الكُمُّ is the subject of a nominal sentence
• واسِعٌ is the predicate of a nominal sentence

In everyday writing, those short vowels are usually omitted, so you normally just see:

• الكم واسع

A careful full pronunciation is:

• al-kummu wāsiʿun

When pausing at the end of the sentence, it is often pronounced more like:

• al-kumm wāsiʿ

A few pronunciation points:

• الـ in الكم is pronounced clearly as al-
• ك is a moon letter, so the l does not assimilate
• كُمّ has a doubled m sound because of the implied shadda
• وا in واسع gives a long ā
• the final ع is the consonant ʿayn, a sound English does not have

Because ك is a moon letter.

In Arabic, the l of الـ is pronounced before moon letters, but it assimilates before sun letters.

Since ك is a moon letter, you say:

• al-kumm

not:

• ak-kumm

So this word keeps the l sound of the article.

It is an adjective in meaning, but in the sentence it functions as the predicate.

That means it is not directly modifying the noun inside a noun phrase. Instead, it is saying something about the noun.

Compare:

• الكم واسع — the sleeve is wide

  • واسع is the predicate

• الكم الواسع — the wide sleeve

  • الواسع is an adjective directly modifying الكم

So the same descriptive word can appear in two different grammatical roles.

Yes, that is a common beginner reaction, because the spelling looks similar.

But here:

• الكم has the definite article الـ
• كم as a question word does not take الـ

So الكم here is a noun, not a question word.

Also, in normal unvocalized Arabic script, several words can look identical until context or vowel marks make the meaning clear. That is very common in Arabic reading.