الكوكب كبير.

A careful MSA pronunciation is al-kawkabu kabīrun.

In normal pause, many speakers drop the final short vowels and say:

al-kawkab kabīr

A few pronunciation notes:

• ال = al-
• كو sounds like ko with a long-ish o
• كب in كوكب ends with a b sound
• كبير has a long ī sound, like ee in see

Also, the l in ال is pronounced clearly here, because ك is a moon letter, not a sun letter.

Because Arabic usually does not use a present-tense verb meaning is/are in simple sentences like this.

So:

• الكوكب كبير = The planet is big

This is called a nominal sentence in Arabic. The meaning is is understood automatically.

If you wanted was, then Arabic would use a verb, for example:

• كان الكوكب كبيرًا = The planet was big

This is the normal pattern for a simple nominal sentence in Arabic:

• subject
  • predicate

So here:

• الكوكب = the subject
• كبير = the predicate describing it

Literally, it is something like:

• The planet — big

which in natural English becomes:

• The planet is big

Excellent question. The difference is:

• الكوكب كبير = The planet is big
  
  • This is a full sentence.
• الكوكب الكبير = the big planet
  
  • This is just a noun phrase, not a complete sentence by itself.

In الكوكب كبير, كبير is the predicate, so it is normally indefinite.

In الكوكب الكبير, الكبير is an adjective directly attached to the noun, so it must match الكوكب in definiteness.

Yes. كبير is an adjective meaning big.

But in this sentence, it is not being used inside a noun phrase like the big planet. Instead, it is functioning as the predicate of the sentence:

• الكوكب كبير = The planet is big

So it is still an adjective, but grammatically it is doing the job of the second main part of the sentence.

Because كوكب is a masculine singular noun, and the describing word must match it.

So:

• كوكب = masculine singular
• كبير = masculine singular

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

For example:

• المدينة كبيرة = The city is big

So the form of the adjective changes to agree with the noun in gender and number.

In fully vowelled formal Arabic, the sentence is:

• الكوكبُ كبيرٌ

This is pronounced:

• al-kawkabu kabīrun

Why?

• الكوكبُ has ḍammah because it is the subject
• كبيرٌ has ḍammah tanwīn because it is the predicate and is indefinite

However, in most everyday writing, these endings are not written, so you usually just see:

• الكوكب كبير

And in normal speech, many speakers pause at the end and do not fully pronounce those endings.

Because short vowels and tanwīn are usually not written in ordinary Arabic text.

So:

• fully marked: كبيرٌ
• normally written: كبير

Both represent the same word here. The second is just the standard unvowelled spelling.

This is very common in Arabic: learners often have to know the grammar in order to know what ending is implied.

Not as the normal equivalent of The planet is big.

The standard basic sentence is:

• الكوكب كبير

Putting كبير first is not the normal beginner pattern for this meaning. Arabic can change word order for style or emphasis in some contexts, but for straightforward MSA, you should learn:

• subject first
• predicate second

So الكوكب كبير is the best basic form to use.

Then you need a noun phrase, not a full sentence:

• الكوكب الكبير = the big planet

Here both words are definite:

• الكوكب = the planet
• الكبير = the big

This is one of the most important distinctions in Arabic:

• الكوكب كبير = sentence = The planet is big
• الكوكب الكبير = phrase = the big planet