المصروف الثابت كبير والمصروف المتغير صغير.

Because this is a nominal sentence in Arabic. In the present tense, Arabic usually does not use a separate word for is/are.

So:

• المصروف الثابت كبير = The fixed expense is big/large
• literally: the expense fixed big

This is completely normal in Modern Standard Arabic. If you wanted a past or future meaning, then Arabic would use a verb.

In Arabic, adjectives normally come after the noun they describe.

So:

• المصروف الثابت = the fixed expense
• المصروف المتغير = the variable expense

This is the opposite of normal English word order, where adjectives usually come before the noun.

A useful pattern is:

• noun + adjective
• كتاب كبير = a big book
• الكتاب الكبير = the big book

Because الثابت and المتغير are attributive adjectives: they are part of the noun phrase and directly describe المصروف.

So:

• المصروف الثابت = the fixed expense
• المصروف المتغير = the variable expense

In Arabic, an adjective that directly modifies a definite noun must also be definite. That is why it also takes الـ.

But كبير and صغير are different: they are the predicate of the sentence, meaning they tell you something about the subject:

• المصروف الثابت كبير = The fixed expense is large
• المصروف المتغير صغير = The variable expense is small

Predicate adjectives in this kind of sentence are usually indefinite, so they do not take الـ.

No. It is not an iḍāfa. It is a noun + adjective phrase.

Here is the difference:

• noun + adjective: المصروف الثابت = the fixed expense
• iḍāfa: something like مصروف الشركة = the company’s expense / the expense of the company

A quick clue:

• In المصروف الثابت, both words can take الـ, because the second word is an adjective.
• In an iḍāfa, the first noun usually does not take الـ.

Because they agree with المصروف, which is masculine singular.

In Arabic, adjectives must agree with the noun in:

• gender
• number
• definiteness

So since المصروف is masculine singular, the adjectives are also masculine singular:

• الثابت
• المتغير
• كبير
• صغير

If the noun were feminine, the adjective would usually be feminine too.

The و means and.

It is written attached to the following word:

• والمصروف = and the expense

This is very common in Arabic. Short words like و are often attached in writing.

So the sentence has two linked parts:

• المصروف الثابت كبير
• والمصروف المتغير صغير

Yes, Arabic can sometimes omit the repeated noun if the context is very clear. But repeating المصروف makes the sentence clearer and more explicit.

So:

• المصروف الثابت كبير والمصروف المتغير صغير
  is very clear and complete.

A shortened version like:

• الثابت كبير والمتغير صغير

might be possible in context, but by itself it is less explicit. For learners, the full version is easier to understand.

Arabic often uses a singular noun to talk about a general category or type of thing.

So المصروف here can mean something like:

• the expense
• the expenditure
• the spending

It is talking about fixed expense versus variable expense as categories.

If you wanted actual plural expenses, Arabic might use a plural such as المصروفات, depending on context.

Although it is written الثابت, it is pronounced with sun-letter assimilation because ث is a sun letter.

So:

• written: الثابت
• pronounced: ath-thābit

The l sound of الـ disappears in pronunciation and the ث is emphasized/doubled.

By contrast:

• المصروف is pronounced al-maṣrūf
• المتغير is pronounced al-mutaghayyir

because م is a moon letter, so the l sound is pronounced normally there.

In full grammatical vowel marking, it would be:

المَصْرُوفُ الثَّابِتُ كَبِيرٌ وَالمَصْرُوفُ المُتَغَيِّرُ صَغِيرٌ

A transliteration would be:

al-maṣrūfu ath-thābitu kabīrun wa-l-maṣrūfu al-mutaghayyiru ṣaghīrun

The main case pattern here is:

• المصروفُ: nominative, as the subject/topic
• الثابتُ: nominative, because it agrees with المصروف
• كبيرٌ: nominative, as the predicate
• and the same pattern repeats in the second half

In normal reading and speech, especially outside very formal contexts, these final case endings are often not pronounced.

Yes, depending on context, that can sound more natural in English.

Literally:

• كبير = big/large
• صغير = small

But with things like expenses, costs, or amounts, English often prefers:

• high
• low

So the Arabic is structurally simple, but the most natural English wording may vary by context.