الدين كبير ونسبة الفائدة منخفضة.

Because this is a nominal sentence in Arabic. In the present tense, Arabic normally leaves out the verb to be.

So:

• الدين كبير = The debt is large
• literally: the debt large

If you wanted was, will be, etc., then Arabic would use a verb such as كان.

Because كبير here is not an attributive adjective inside a noun phrase; it is the predicate of the sentence.

Compare:

• الدين كبير = The debt is large
  
  • كبير is the predicate
• الدين الكبير = the large debt
  
  • الكبير is an adjective describing the noun directly

In Arabic, predicate adjectives in sentences like this are usually indefinite, so they normally do not take ال.

Because Arabic adjectives and predicate adjectives must agree with the noun they describe.

• الدين is masculine singular → كبير
• نسبة is feminine singular → منخفضة

So the agreement is:

• الدين كبير
• نسبة الفائدة منخفضة

This kind of gender agreement is very important in Arabic.

It is an iḍāfa construction, often called a construct phrase.

Structure:

• نسبة = rate / proportion / percentage
• الفائدة = the interest

Together:

• نسبة الفائدة = the interest rate

Literally, it is something like rate of the interest.

In an iḍāfa:

• the first noun usually does not take ال
• the second noun is in the genitive
• the whole phrase can still be definite

So even though نسبة has no ال, نسبة الفائدة is definite because الفائدة is definite.

Because in an iḍāfa, the head noun is the first noun.

Here, the thing being described as low is the rate (نسبة), not the noun interest (الفائدة) by itself.

So Arabic treats it like this:

• [interest rate] is low

And the adjective agrees with نسبة, which is feminine singular, so you get منخفضة.

It can mean either one in unvowelled writing, but they are actually different words:

• الدَّيْن = debt
• الدِّين = religion

Without short vowels, both are written الدين.

In this sentence, the context makes it clear that it means debt, especially because the second clause talks about interest rate.

Here it is pronounced ad-dayn.

That is because د is a sun letter. With sun letters, the ل of ال is not pronounced separately; it assimilates to the next consonant.

So:

• written: الدَّيْن
• pronounced: ad-dayn

But in الفائدة, the letter ف is a moon letter, so the l sound is pronounced:

• al-fā'ida

A fully vowelled version is:

الدَّيْنُ كَبِيرٌ وَنِسْبَةُ الْفَائِدَةِ مُنْخَفِضَةٌ

The case endings are:

• الدَّيْنُ — nominative
• كَبِيرٌ — nominative
• نِسْبَةُ — nominative
• الْفَائِدَةِ — genitive, because it is the second term in the iḍāfa
• مُنْخَفِضَةٌ — nominative

In normal reading or pause, many speakers do not fully pronounce all these endings.

Because و (and) is a clitic in Arabic. It is written attached to the following word.

So:

• ونسبة = and the rate

This is standard Arabic spelling. The same happens with other short particles too.

Yes. كبير literally means big/large, but Arabic uses it naturally with abstract things too, just like English does in phrases such as a large debt.

So:

• الدين كبير = the debt is large / high

This is completely natural Arabic.

Yes, Arabic has some flexibility in word order. The version you have is a very normal, straightforward order:

• الدين كبير
• ونسبة الفائدة منخفضة

That said, Arabic can sometimes move the predicate first for style or emphasis. But for learners, the pattern in your sentence is the most useful basic one.

Then you would be making noun phrases, not full statements.

You would say:

• الدين الكبير = the large debt
• نسبة الفائدة المنخفضة = the low interest rate

Notice the difference:

• الدين كبير = The debt is large
• الدين الكبير = the large debt

So the presence or absence of ال on the adjective helps show whether it is:

• part of a description inside a noun phrase, or
• the predicate of a sentence