القمر كبير.

In Cairene Egyptian, you would usually hear something like el-ʾamar kebīr.

A few useful notes:

• ال is commonly pronounced el- in Egyptian speech.
• ق is often pronounced as a glottal stop ʾ in Cairo, so قمر sounds like ʾamar.
• كبير is commonly pronounced kebīr.

In some other Egyptian accents, ق may sound more like g, so you might also hear el-gamar kebīr.

Because Arabic usually leaves out the verb to be in the present tense.

So القمر كبير literally looks like the moon big, but it naturally means The moon is big.

This is very normal in both Standard Arabic and Egyptian Arabic. If you wanted to say was or will be, then Arabic would use a verb.

Arabic adjectives normally come after the noun, not before it.

Also, in this sentence كبير is functioning as the predicate of the sentence:

• القمر = the subject
• كبير = what is being said about it

So the pattern is basically subject + predicate, which is very common in Arabic.

Because this sentence means The moon is big, not the big moon.

That distinction matters in Arabic:

• القمر كبير = The moon is big
• القمر الكبير = the big moon

In other words, when the adjective is part of a full sentence and acts as the predicate, it is usually indefinite. When it is directly describing a definite noun inside a noun phrase, it also becomes definite.

Yes. In Arabic, adjectives match the noun in gender and number.

Here:

• قمر is masculine singular
• so كبير is also masculine singular

If the noun were feminine singular, you would usually use كبيرة instead.

Because ق is a moon letter.

With moon letters, the l in ال is pronounced clearly, so you get el-ʾamar, not a doubled next consonant.

Compare:

• القمر → el-ʾamar
• الشمس → esh-shams

In fact, قمر is the word that gives the name moon letters.

It is fine in Egyptian Arabic too.

The main difference is usually pronunciation, not the basic structure. The written sentence القمر كبير works perfectly well, but an Egyptian speaker would pronounce it in an Egyptian way, such as el-ʾamar kebīr.

So the grammar is not especially formal here; it is a very basic, natural sentence pattern.

Because normal Arabic writing usually leaves out most vowel marks.

So you see:

• القمر كبير

rather than a fully marked version.

Also, in Egyptian Arabic, speakers do not use Standard Arabic case endings in everyday speech. So a learner should focus more on recognizing the word shapes and learning the spoken pronunciation than on expecting every vowel to be written out.