المحيط كبير.

Because Arabic often leaves out the present-tense to be in simple statements.

So المحيط كبير literally looks like the ocean big, but it means the ocean is big.

This kind of sentence is called a nominal sentence in Arabic. It is built from:

• المحيط = the subject
• كبير = the predicate

If you wanted was or will be, Arabic would use an actual verb, such as كان or سيكون.

It is a nominal sentence: a sentence that starts with a noun rather than a verb.

In traditional Arabic grammar, the parts are:

• المحيط = المبتدأ (the topic/subject)
• كبير = الخبر (the comment/predicate)

So the structure is basically:

• [subject] + [description]

Here, the sentence is simply saying something about المحيط.

In Arabic, the word that tells you something about the subject usually comes after it.

So:

• المحيط كبير = The ocean is big

This is normal Arabic word order for this kind of sentence.

Also, if كبير were directly modifying the noun inside a noun phrase, it would still come after the noun:

• المحيط الكبير = the big ocean

So Arabic adjectives normally follow the noun, whether they are part of a noun phrase or functioning as the predicate.

Because here كبير is the predicate, not part of a noun phrase.

Compare these:

• المحيط كبير = The ocean is big
• المحيط الكبير = the big ocean

In المحيط كبير, the first word is definite (the ocean), but the predicate is usually indefinite. That is the normal pattern in sentences like this.

So:

• المحيط = definite
• كبير = indefinite predicate

That difference helps show that this is a sentence, not just a noun phrase.

Because it matches المحيط, which is a masculine singular noun.

In Arabic, adjectives and predicate adjectives usually agree with the noun they describe in:

• gender
• number
• often definiteness when they are attributive adjectives

Here, since المحيط is masculine singular, the matching form is كبير.

If the noun were feminine, you would expect كبيرة instead.

The fully vowelled form is:

المحيطُ كبيرٌ

Those endings show that both words are in the nominative case, which is expected in this kind of simple nominal sentence.

In ordinary Arabic writing, these short vowels are usually omitted, so you normally see:

المحيط كبير

When reading aloud in pause, many speakers also do not pronounce the final case endings.

It is pronounced roughly al-muḥīṭ.

The ال does not assimilate here, because م is a moon letter, not a sun letter.

So you pronounce the l clearly:

• al-muḥīṭ

Compare that with a sun-letter word such as الشمس, where the l is not pronounced as a separate sound.

Yes, but it would usually mean something different.

• المحيط كبير = The ocean is big
  A complete sentence.

• محيط كبير = a big ocean
  A noun phrase, not a full sentence by itself.

So the presence or absence of ال changes the grammar and meaning.

The normal, neutral order is:

• المحيط كبير

That is the most natural form for a beginner to learn.

Arabic can sometimes change word order for style, emphasis, or literary effect, but for standard straightforward statements, keeping the subject first and the predicate second is best.

So for everyday MSA learning, treat المحيط كبير as the standard pattern.

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

That means it is not just attached to the noun inside a noun phrase; instead, it is telling you something about the subject.

Compare:

• المحيط الكبير = adjective inside a noun phrase: the big ocean
• المحيط كبير = predicate adjective in a sentence: the ocean is big

So the same word can appear in both roles, but the structure changes the meaning.